Hi matthersoft,
I would prioritize the mattress over the support system because that’s the part of your sleeping system that will have the biggest effect on PPP. The Laxeby will be fine with any foam mattress in terms of support (as long as the manufacturer doesn’t insist on their own foundation for warranty purposes) but I would rather use it under a mattress which is too thick to take full advantage of the adjustable slats but otherwise works well for you than use it under a mattress that is thinner but doesn’t work as well for you just for the sake of being able to use the base for fine tuning. It will have some effect on PPP but with a thicker mattress the effect will be much smaller.
I would treat the thickness of your mattress as less important than how well the mattress works for you and the quality of the materials inside it.
Beloit would certainly be well worth visiting and they are a member here which means that I believe they compete well with the best in the industry. If you let me know your city or zip code I’d be happy to let you know of any of the other options or possibilities I’m aware of in your area.
My mattress was custom built specifically for us and wouldn’t likely be suitable for someone else. You can see the details here. There are too many unknowns, variables, and personal preferences involved in a mattress choice for anyone to be able to make specific mattress suggestions for someone else unless it is an online retailer or manufacturer who is helping you choose one of the models they make that your conversations indicate has the best odds of success (see mattress firmness/comfort levels in post #2 here). I’m happy to help with “how” to choose but the specifics of “what” mattress to choose is should always be based on your own personal testing or more detailed conversations with an online retailer or manufacturer.
I would want to know the quality/density of all the layers in a mattress so you can identify any weak links in the mattress and make more meaningful comparisons to other mattresses. The only layer in this mattress where the density is listed is the 1.5" of 3 lb memory foam which is a low quality material which in combination with the other unknown layers (some of which are also very likely to be lower quality/density as well) would mean that I would avoid it (see guideline #4 in the mattress shopping guidelines). You can read more about Night Therapy mattresses in post #2 here and the links it includes but in general I would tend to avoid them because of the lower quality/durability materials they use.
Phoenix