Hi ddeg10,
Welcome to the Mattress Forum! 
It is an unfortunate truth that too many “review” sites for mattresses are simply “revenue” sites set up to generate click-throughs and affiliate revenue from mattress companies, with no objective meaningful information being given to assist consumers besides a 4-5 “star” rating, and often making the mattress companies the most visibile on their “review” sites who are providing the highest affiliate fees . Of course, the fact that a web site generates income in and of itself isn’t an indicator that the information presented is necessarily biased, as that would be pure cynicism. The web sites aren’t put up as charitable work, including mine, so what you have to do is look at the information being presented and how it is presented and the level of education and objectivity on the site, and determine if the site is helping to instruct you to find your own product or if it is choosing something for you. And this is one of the areas where The Mattress Underground has earned the stellar reputation that it currently holds. You can see the core values of the site here. I’m glad you’ve found us!
You’re probably correct that your repositioning has something to do with the gradual loss of comfort in your current mattress. I would also have a concern with our wife sleeping upon her stomach that the mattress may be allowing her to sink in a bit more than is appropriate for someone sleeping in the prone position.
The two basic functions of a mattress are to provide deep support / alignment, and then to provide comfort (see this article). If you’re prioritizing between the two, I would definitely recommend alignment/support first (especially with your wife sleeping in the prone position), and then “just enough” surface comfort to get the job done. Different sleeping positions require, in general, different levels of surface plushness, as described in this article. You need to be especially careful with someone sleeping prone so that the mattress doesn’t allow too much sag on top to accentuate the forward (lordotic) curve in the lower back, which can be quite uncomfortable if that happens. This can be a problem with memory foam mattresses especially, as the memory foam itself in the upper layers does not have much resilience (“pushback”) and does allow you to sink in, and this can become more of an issue over time as the memory foam softens, along with the polyfoam support core beneath it.
As for “medium” or “medium-firm”, there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science. Ultimately, there really isn’t a reliable way to match one mattress to another one in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences) based on the specifications of the mattresses (even assuming that you can find out all the specifications you would need for both mattresses you are comparing in the first place), and your own personal testing will be the most accurate indicator of the potential for an item to meet your specific needs, provided that the componentry used within a mattress if of a good quality.
To help select a mattress, the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
You may also want to read post #3 here, as it has more information and suggestions about higher BMIs and selecting a mattress.
The Nectar uses a 1" of 4 lb gel memory foam, a 3-inch 3.5 lb memory foam layer, and finally a 2.2 lb polyfoam core. They don’t list the thickness of the polyfoam core, or the thickness of the quilted gel memory foam panel or the density of that material. The polyfoam core uses a good density foam, but I would use caution with the 3" of 3.5 lb. memory foam on top of the core, as that is lower than I would recommend (see the durability guidelines here ). They are a product assembled in the USA of components sourced from China and the US. They do not specify what is from where. I would caution against purchasing this item before knowing the complete specifications and component origins. I would read post #6 here about mattresses and materials imported from Asia or China and which may have been compressed for long periods of time in either shipping or storage before being purchased and being sourced in China would make this somewhat of a risky purchase IMO. Additionally, I’ve heard from people within the industry of some fulfillment issues with Nectar, but this may be addressed by now.
You can read more about the specifications of the Leesa in the Simplified Choice Thread here. I’d advise caution because of the 2" of 3 lb memory foam as a weak link in this mattress.
The Best Mattress Ever (you can also see the specifications of their mattresses in the Simplified Choice thread here) would use the best quality material as compared to the other mattresses you mentioned. They are also a site member here, which means that I think highly of them and their products.
Regarding reviews, and as I alluded to earlier in this post, while other people’s comments about the knowledge and service of a particular business can certainly be very helpful, I would always keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about using anyone else’s suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
While I can certainly help with “how” to choose, it’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
The best advice I can offer is to follow the steps outlined in the mattress shopping tutorial that I linked to earlier in this reply, and then analyze the specifications of any product you are considering. If you are considering shopping online, and while certainly not a complete list of what is available, you may wish to consider the experience and expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of latex and memory foam and other options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
If you have any specific questions about any products you are considering after reading through this information, I’ll be happy to do my best to be assistive.
Phoenix