Hi bknox3,
Welcome to the Mattress Forum! 
Your question is common, with many people trying to decide between the many boxed-bed offerings available.
The first thing to focus upon is learning about mattress componentry and how to go about selecting a mattress. To do that, I recommend that you start by reading 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).
While again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … 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.
I agree with you 100%. 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).
[url= https://mattressunderground.com/mattresses/style-preferences-and-statistics/your-sleeping-positions.html]Here’s a good article[/url] that speaks to different sleeping postures and things to look for.
You’ll always want to check with each manufacturer as to what is appropriate for their mattresses, not only for support but also to maintain a valid warranty. Most manufacturers recommend some sort of a solid foundation, with slats spaced anywhere from 2" – 5" apart (it varies by manufacturer and model). A true “box spring” would be an active unit with coils that flexes, and generally would be contraindicated by the manufactures you mentioned. There’s much more about foundations and the differences between them in this thread.
All of the brands you mentioned have been discussed quite a bit here on the forum (Brooklyn Bedding and Novosbed are site members here, which means that I think highly of them). If you want to perform a forum search on these or any other brand, just click on this link and enter the name of the brand you might be curious about.
Boxed-bed “Simplified Choice” mattresses are discussed in greater detail in this thread. The specifications for the Tuft and Needle, Casper, Leesa Brooklyn Bedding Signature, and Novosbed are all listed in that thread.
If you have more specific questions after reading through this, feel free to post them back here and I’ll do my best to be assistive.
Phoenix