I might still be too young to really notice, but I did have major back surgery as a teenager and am a little sensitive to beds. Like many things, I think there's extreme diminishing returns, especially at "traditional" mattress stores. From the research I've done online and my personal and family experience - I suggest if you're under 250lbs and can handle the heat of memory foam - buy a bed in a box memory foam online for 1/3 or less the cost of a upscale or major in person brand. I'd even avoid the "branded" casper / tuft and needle / etc and go for a decent rated Amazon special or Members Mark from Sam's Club. You don't need or want a box frame with these, so get or make a cheap platform.
If you're heavier I'd recommend a combo - either bought in one mattress or one you create yourself - with individual wrapped coils as a base and 2-3 inches of memory foam on top. Saatva makes an all in one at the higher end of what I'd want to pay, there are other options - just make sure the coils are individually wrapped. This made a huge difference 20 years ago for my back, and minimizes movement transfer as well I think. I've seen mixed info on these latest hybrid mattresses as to if you need a box spring or not, I've only used older style with a box spring so IDK how well it works skipping that.
The major benefit IMHO of the online sales is most of them give you at least a 30 day, some up to a year, return period if it doesn't feel great how you use it "in real life". I never find I can tell about a mattress in 10 minutes in a store, and it's not like even the first couple days at home really let me know. The major downside is that you need to be able to handle getting the old bedding out and new in yourself, with a friend, or pay extra for the service if it's even offered. That said, even if I couldn't or didn't have family or friends who could bang it out, I'd pay the couple hundred to a handyman for hire to pull it out and put it in for me, or the service because again, you get better trials, and the overall savings is huge. Well, and most mattress stores also charge a delivery and haul away fee now anyway so...