Buy it for Life

Check out Naturepedic mattresses. 100% natural materials except the steel springs. They have a modular design that you can open up occasionally to flip the springs or topper if you like. You can also replace the individual components as desired, not that we've needed to. Very comfortable too, although during the trial period we ended up switching out the foam topper for one that was less firm than we liked on the show floor (they allow this for free within a certain time frame after buying).

I hear you on the memory foam but hear me out. I want something firm but still very comfortable that won't hurt from hardness in the morning. I like some support but not a ton and I hate sinking into a mattress like foam.

My old mattress crapped out about 5 years ago. I bought and returned several mattresses until I settled on the following. I have a fairly cheap firm mattress from a local store. Firm are normally cheaper by the way. I think it was 650 for a king. Then I bought a really nice 3" closed cell foam topper for $200. I expected it to only last a few years, but it's still great. At least another couple years left. But the mattress is still like new because there's zero cushion on it.

Every mattress will wear. But it's primarily the cushion and weak springs that wear. So BFL is not reasonable but you can extend the life in certain circumstances. Since firm mattresses have hard springs and no cushion, they'll last a lot longer. The topper is essentially the only real wear item. Since they come in different materials and thicknesses, you can dial in your preference easily.

I got a Purple Mattress: you get the stability of spring coil but the softness of memory foam. I really like it.

Here's a relatively short video of a real-world review. This guy also later reviewed a Nectar mattress, which is memory foam, but he didn't like it and returned it.

Sounds like you’re not keen on futons. But worth noting that some futons can be custom made with the materials you want, and you can have a summer side and winter side. The summer side might have coconut choir or something highly breathable. It’s not BifL, but at least you can get something with biodegradable materials.

When it came time to replace a mattress that was older than me, I did a maddening amount of research before making a purchase, since they're pretty darn expensive. Here's a few takeaways:

• Mattresses at physical stores have a significant markup compared to online mattresses (there's also a comedic level of public rivalry between physical mattress store owners and online store owners on mattress subreddits, they absolutely hate each other's guts. Be wary of those subreddits by the way, it's difficult to get truly objective information there due to how astroturfed they are).
• Cheap memory foam mattresses wear out pretty quickly, and are very hot to sleep on.
• Full foam mattresses, especially full Latex ones, are incredibly heavy, and very difficult to change the sheets on, all for very little, if any, real advantages.

Of the better things to look for in a mattress, I would recommend:

• Hybrid Coil with Latex Foam mattresses. These combine the best of both worlds, and are what I personally chose in the end. Latex foam lasts an incredibly long time, is much cooler than memory foam, and never really develops low spots like memory foam does. It should last the entire usable life of the mattress, which could be as much as 10 to 15 years.
• Try to opt for a pocketed-coil spring if the budget allows for it (yours certainly would). Pocketed coils contour to your body better, and transfer much less bouncy energy across the bed to a partner compared to traditional interwoven coil mattresses.
• While it usually costs a slight premium, if the budget allows, try to opt for a mattress that uses non-plastic based materials to reduce the chance of microplastics ending up in the environment when the mattress becomes end of life (there are recycling facilities for mattresses, but I'm not sure if they re-use/melt down the polyester outer shell).

Personally, I purchased my hybrid mattress online at Arizona Mattress, since they had the lowest price for the materials and design that I wanted. I've been sleeping on it for nearly 2 years now, and have been very pleased with it.

If you search around for a coupon code, you can bring down the price another 5 or 10% or so. I got a coupon code from this mattress manufacturer coalition forum, which is such an odd thing to exist, lol. That forum has some affiliate coupon codes for almost all of the major online mattress companies.

If possible, try out a mattress first at a local store, then purchase it online to avoid the markup.

If you're open to trying a latex mattress, sleeponlatex.com is the best mattress you can find. Latex mattresses can last a very long time (10 to 20 years), and theirs is great quality.

My partner and I had tried so many different kinds of mattresses when we were hunting, and theirs was the first that was comfortable, soft, supportive and breathable. We got a medium firm mattress with a soft 3" topper.

They have a 100 day return policy for their mattresses so there was no risk for us trying it!