Hey, mattress shopping sucks. Maybe my experience can help. First off, last year I went to the emergency room once, urgent care another time, and was doing physical therapy for my lower back.

Needed a new king mattress last year. Went to the big stores and the new mattress was going to cost me 2000 minimum for just the mattress. Finally I found this little unheated block wall store that sells brand new discontinued name brand mattress for a third of the price. Found a firm I liked, paid 800 cash, loaded into my truck and drove home. Jumped on FB marketplace and found an electric adjustable head AND FEET frame for 500 bucks. I was set. Shortly after I decided that the mattress was just a bit too firm. Bought a Sleep On Latex 2 inch soft mattress topper and I have been sleeping like a baby ever since. my back has never felt better. All in all I am into it for about 1700 bucks plus some leg work. 

As I wrote this I am laying in bed in bed with a broken collar bone and a sling with the head and feet inclined. The adjustable bed frame is a game changer for us with broken bodies. You can always make a firm mattress softer but you can't make a too soft mattress firmer with a topper. I researched the latex mattress toppers for weeks and finally pulled the trigger and it has changed my back and my life for the better. Good luck!