Building a mattress for picky people

Hello! I think I’m almost ready to pull the trigger and order my latex mattress.But since I can’t get out and really test what they feel like, I could use some help.

Here are our details:
Husband is 6’1" 280 lbs, back sleeper, chronic insomniac with a bi-pap and a variety of prescription sleep meds Does not complain of any pain upon waking
I am 5’6", 170 lbs, mostly back sleeper, but I do like to roll onto my side. My time on my side is short b/c it hurts. Don’t know whether this is my current mattress or my aging body or both. I wake with pain in the muscles of my mid-back and around my hips. I sleep with a low pillow under my knees to reduce my lumbar curve.
we are 54 and 53.

Mattress preferences: we have had an original Tempurpedic for 24 years, avoiding replacing it b/c current memory foam all feels “soupy.” What we like about the old TP is that it is firm to the touch, like wet sand, that we are very much sleeping “on” rather than “in” the mattress. I require total motion isolation. I go to bed first and like that I do not feel my husband get into bed.

I’m anxious about DIY bc I don’t want to mess it up, but also worry that pre-made mattresses are just going to be too squishy for my taste. So I’d love a little guidance on getting it right. Thanks!