Hi H8tobogie,
There’s a lot of information on this site that you will find valuable with your approach. Since I’ve read much of it, I often try to restate some important pieces that Phoenix and others have written. A note about building a DIY mattress… It often boils down to trial and error if you’re not modeling your mattress on one that a company has already made and you’ve found you like. Phoenix notes it’s where the science and art of mattress design meet.
DIY thread
https://forum.mattressunderground.com/t/foam-factory-any-good
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There are several places to get zip covers. The above link has several. A mattress cover may also include wool or other materials to change how the mattress feels.
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You’re into the realm of layering toppers and how they’ll interact together… Which is where mattress design becomes an art. You’ve got high quality versions of latex, buckling gel, and wool. You’ll certainly have nearly every possible combination that you can try
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There was recently some dicussion on buckling column gel, and intellibed. I actually just tried it. Nevertheless, I have no idea what it would do under another layer.
https://forum.mattressunderground.com/t/intellibed-and-other-buckling-column-gel-beds-for-pain -
If your only problem is your shoulders causing a posture or pressure problem, you might consider a zoned bed (further complicating your build, since you could also zone a DIY bed). Flobeds vzone is the most recent one I read on this thread, but I trust there are many others. Basically, at your shoulders youd have a zone that is softer, for your shoulders to sink deeper and with more pressure relief.
My personal 2 cents… It’ll be essentially random good luck if the mattress you’ve outlined works. You’ve selected all high quality materials, but how they’re going to work as layers is a huge unknown, and it’s definitely not something you can try ahead of time. On the plus side, you basically have 1 of everything so by rearranging the layers you have more chances that one of them feels just right. Budget and time wise, I’d therefore suggest you plan for some suplus $$'s / and or plan for additional time to replace some of the layers or make adjustments after you make your first version of your mattress.