I’m a big DIYer, and the most crazy sounding DIY thing I’ve done is build my own mattress. I’m not sure how I came across DIY mattresses, but I started researching it after looking at mattress prices in the area.
There are several sites where you can buy mattress components, but the most popular seems to be Arizona Premium mattress. The best recommendation I have seen is to get three 3” layers of latex from firm to medium to soft. If you get the top layer too soft, you can just swap the layers to firm it up. There are tables that recommend firmness levels based on weight, body type, sleeping position, etc. Just buy the layers you want and a mattress cover you want and zip them up into the cover. Latex is supposedly the premium mattress material even over memory foam because memory foam retains heat and breaks down quicker over time. Arizona Premium Mattress has coil springs too if you rather go that route.
I got a split kit from them and got different firmness for each side of the bed for me and the wife. I think spent $1400 for what would have cost a lot more in store. If the top layer wears out ten years from now, I just have to replace the top layer and/or mattress cover.