I have had mild back pain all my life and often felt stiff getting up in the morning. A while back I moved to a yoga mat (both to be more minimalist and because I thought it would help my back) and missed the padding for a few days, then I was fine. I then started sleeping on the floor with only a sheet under me and it now feels even better. The only two things I feel I need are a normal sized pillow and I do like to use one of those large bamboo pillows that you hold between your legs while on your side.
Right now when I sleep I feel some sort of relief as my back flattens against the hard floor.
For naps or lounging like sitting on a couch reading with a dog, I like them really soft.
What about you guys?
You might find that you don't even need the pillow so much as something that will support your spine in proper alignment.
Something like these might work just as well for you:
https://www.amazon.com/Bamboo-Block-inches-Rectangular-Pillow/dp/B00ML1BTSS
https://www.amazon.com/Therapeutic-Hinoki-Cypress-Shoulder-Relaxation/dp/B06XDJ49QG