Hi lowbmi,
Welcome to the Mattress Forum! 
I’m sorry you’re having difficulty finding something that is plush enough for you. A topper itself can only do so much in changing comfort, as it works in toto with all of the layers in the sleep system, and if the componentry beneath it wasn’t close enough to your comfort preference to begin with, and you are very specific in the plushness you desire, it’s unlikely a topper itself would provide you enough relief.
As for temperature issues, the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here. All foams are insulators to some extent, and a softer mattress that you sink “into” more that sleep “upon” can also sleep warmer, regardless of the materials used.
I’m sorry your Luxi isn’t working out for your personal comfort preference. I’m not sure why the mattress would “dip” in the center, unless you had a split configuration and the top layers are not pressed into each other solidly within the encasement. While you say there is “nothing wrong” with you foundation or bed, I am curious what you are using for both.
While “simplified choice” mattresses are designed to work for the broadest range of the sleep spectrum, it wouldn’t be correct to make a blanket statement about ideally what “most” memory foam mattresses work for, as there are plenty of high and low BMI individuals who enjoy memory foam mattresses, as long as the componentry is of adequate density/quality, as the selection of a mattress has a large personal subjective preference to it.
All the layers of a mattress work together, and someone at 100 pounds certainly would “sink down” into 14 ILD Talalay latex, but whether or not this would be to your personal preference of course would depend upon the other layers of the mattress as well and your own tolerance for plushness. Latex will be the most resilient (“bouncy” or buoyant) foam, but you would notice more of that with the higher density/firmer latex layers than with the softer layers, but it would still be superior to polyfoam or memory foam.
Yes, I understand it can be frustrating with certain somatotypes/BMIs/comfort preferences to find something that works. While not normally the first recommendation I make, zoning systems of various types can sometimes be useful and worth considering for people that have more difficulty finding a mattress with the right “balance” between comfort/pressure relief (under the shoulders especially) and support/alignment (under the hips/pelvis especially) or who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article, in post #11 here, post #2 here, and post #7 here and the additional posts they link to but once again the only way to know whether any specific mattress (zoned or otherwise) will be a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) will be based on your own careful testing and/or your own personal experience. Site member Flobeds has a unique V-Zone system and they are very experienced in assisting people who have a difficult time finding a comfortable mattress. They may be worth a phone call.
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
I wouldn’t place too much stock in other people’s opinions of what might be “soft enough” for you, as 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). And as far as returning, I’d place the same amount of stock in those opinions, as most online manufacturers are asking that you donate their product as opposed to returning it if a refund is authorized, and as Ari commented previously, Nest, like most other companies, will work with you to help with a pick up if you can’t find a donation center in your area. I’ve not run across wide-spread issues with returns from Nest Bedding or most of the other major “simplified choice” manufacturers here on the forum or in my discussions off of the forum.
As you may be attracted to the idea of designing and building your own DIY mattress out of separate components, then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project, the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
Unfortunately, I can’t feel what you feel and I can only help with “how” to select a mattress, not “what” to choose. There are simply too many personal variables involved that make it too difficult to accurately select a mattress for someone else. There is no algorithm that could possibly take into account the personal unknowns, individual body types, sleeping styles, personal preferences, somatotypes, health issues, differing masses, levels of fitness and flexibility, etc. Researchers have tried, but it’s just not possible. Post #15 here does talk a bit more about softness, pressure relief and alignment and how they relate and can be perceived differently by various individuals.
Overall, regardless of BMI, I’ll always recommend that you start out be reading through the mattress shopping tutorial here. 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 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.
Assuming that the materials in a mattress you are considering are durable enough for your body type and meet the quality/durability guidelines here relative to your weight range … the choice between different types and combinations of materials and components or different types of mattresses are more of a preference and a budget choice than a “better/worse” choice ( see this article ). The best way to know which type of materials or which type of mattresses you tend to prefer in general terms will be based on your own testing and personal experience because different people can have very different preferences.
As far as appropriateness, I wouldn’t be able to predict if one combination versus another would allow enough of a comfort cradle for you while still be able to provide adequate support, or if using a polyfoam support core would be personally preferable to a latex or innerspring unit. That information can only be accomplished through your own personal testing. Lighter body types tend to do well with softer foams (in comfort layers and support layers) and what feels firm for a lighter body type can feel soft for someone who is heavier. Each person will also sink into the mattress differently depending on their weight distribution and body shape (where they carry most of their weight and the “curviness” of their body)
If you do decide to look online, then you may wish to use 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, even for someone with a lower BMI. 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.
I know that’s a lot of information to process, but you made quite a few statements and brought up many questions, and I wanted to take the time to address them as best I can.
Phoenix