I went through a similar process recently, and ended up getting the Spindle: https://spindlemattress.com/. It's also an "assembly your own" latex setup, although instead of shopping for layers individually, they sell 3-layer sets. I'm pretty happy with it.
I liked that the layers stay separate - you can break the mattress down to make moving easier, and you can replace layers individually, if you ever feel the need (with a 30% discount if you buy from Spindle again inside 25 years).
I liked that the layers stay separate - you can break the mattress down to make moving easier, and you can replace layers individually, if you ever feel the need (with a 30% discount if you buy from Spindle again inside 25 years).