Mattress replacement advice needed!

Hoping to get some advice as mattress buying seems to be extremely difficult, thank you in advance. My current situation is I have a Brooklyn Bedding Bestmattressever that is about 8 years old, it’s the “soft” level. I have 2 twin XL’s to make a king size, my wife sleeps on the other one. Neither of us have every been really happy with them, I suspect we should have ordered a more firm version as we are both side sleepers. The first few years they were ok, but they seem to have been breaking down where we sleep, and we do rotate them about once a year. We both have been developing significant low back pain while in bed, and in the morning after waking up. I tried to fix this by adding a 3" latex topper from Pure Green (their “soft” level), and it did help for a while, but in hindsight I think what we needed was more firmness underneath, not less firmness over the mattress.

I experimented last night and flipped the BB mattress upside down and slept on the very firm polyfoam base with the 3" latex topper over it. The difference was with my prior setup I would feel back pain when laying on my side AND moving, but with the upside down setup felt no pain at all when laying down, but on moving had worse pain. I actually slept pretty well on it last night with the exceptions of any time I had to move. I get that the poly foam is completely not meant to be a top layer, but was only experimenting with something more firm.

My question is what mattress should I consider if I want to continue using the 3" latex topper? I suspect I should consider something more firm to support the spine, and also to work well with the soft topper. I’m also a hot sleeper so would prefer something cool. Finally, as a side sleeper I have very broad shoulders and have found the combination of the 3" latex topper and a fairly high latex pillow work perfectly and I have zero shoulder, head, neck pain and would like to keep that combination while solving the low back/side sleeping issue.

I’ve read a lot of information, some seems to point back to old school Bonnell spring mattresses, but I have an adjustable bed frame (although I have never seen the benefit of that during sleep) so am not sure if I am limited to newer spring types. I’m gravitating towards some sort of hybrid with only a thin (or no) foam topping but not sure where to start.

Companies I’ve considered:
Brooklyn Bedding: I was highly interested in their athletic mattresses but am VERY highly off put by their warranty which is 1.5" of divot AND their warranty states you have to send your mattress back to them on your own dime, making this a useless warranty. Also not impressed with the performance of their mattress, although I’m willing to say that may be due to my mistake of buying the soft version. But there is still significant depression where I sleep, just not enough to qualify for the warranty.

Boring Mattress: Seems really good even though they don’t post much information about materials or specs, but many reviews said it wasn’t that firm so not sure if I’m just going from a soft bed to another soft bed. They do have a hell of a trial period and warranty though, if they offered a “firm” model I’d be all over it.

Costco: Tempted primarily because of Costco’s famous warranty, but overall mattresses are very expensive and seem to have the typical marketing overpricing of the big box brands.

Sunrising: They have a few beds on Amazon with thousands of great reviews and have very good pricing. There are a few other Amazon brands as well such as Linenspa which seem to have great reviews.

Finally price, would love to find a Twin XL for under $600 but if something is just incredible and much better than the competition am willing to go up to $1k, in particular if it has great longevity and has a great warranty.

Edit: I’m 5’9" and a muscular 205lb with broad shoulders but narrow hips. My wife is 5’1" and a fit 110lb but has a very large chest which I feel might be relevant to her sleep choices. Both of us are primarily side sleepers, occasional back, and never stomach.