Hi there SleepingBunny,
Welcome to the MattressUnderground. We are happy to have you here and thanks for the question.
Based on what you’ve described, I would be careful about equating “firm” with “better.” What you’re experiencing may be less about needing a firmer mattress and more about needing a more supportive mattress that doesn’t allow you to sink excessively. Primary support is going to come from your support layer and you want to be mindful of how much foam (of any type) rests on top of it. Too much foam and you may not engage your support layer, even at 250lbs. I am 6’ 220 (former over 250) and my mattress has a very robust support layer with only 2" of 36ILD comfort layer. Granted, I do not have all of the other issues you are contending with.
At 250 lbs, a lot of softer memory foam can let your heavier areas settle too deeply, which can create the feeling of rolling into a hole, struggling to reposition, sleeping hot, and waking up with more aches and pains. Given that you already know you dislike the feel of memory foam, I would probably avoid mattresses that rely heavily on thick memory foam comfort layers, specifically viscoelastic memory foam (tempur style foam).
Your shoulder and hip measurements suggest you still need pressure relief as a side sleeper, especially with fibromyalgia and Ehlers-Danlos. Sleeping comfortably on the floor with a cushion under your hip is interesting, and speaks to what I mentioned earlier, because it suggests you may prefer a very supportive surface with targeted pressure relief rather than a mattress that lets your whole body sink.
If your budget is under $2,000 for a twin, I’d be looking at higher-quality latex hybrids, traditional latex mattresses, or firmer pocketed-coil mattresses with comfort layers that are easier to modify. Latex tends to sleep cooler, is more responsive than memory foam, and many people who dislike the “stuck” feeling of memory foam find it much easier to move around on. Just keep in mind your mattress protector can make or break the hot feel when you are sleeping. Wool filled quilted mattress protectors with a cotton cover or dual sided organic cotton with a thin tpu breathable layer will work better to keeping you cooler feeling sleeping through the night. Coordinated with sheets that are very breathable and natural, like lower thread count cotton, or something like @BalooLiving linen sheets are excellent choices for your mattress, pillowcases and top sheet.
Companies like @EngineeredSleep @DLX, and some of the regional stores and manufacturers in the Pacific Northwest like @PlankAndCoil & @Shepherds_Dream may be worth investigating. Given your combination of side sleeping, chronic pain conditions, and higher body weight, companies that offer comfort exchanges or layer customization rather than trying to guess the perfect firmness on the first attempt can be helpful. That said, all of the Trusted Members here at TMU have seen, heard, and accommodated it before and are very good at targeting folks with the most challenging of situations. You may see them once every 5 or 10 years but they see you everyday.
The good news is that a twin mattress gives you a lot of room in your budget. You should be able to find something substantially better than your current setup without needing to spend the full $2,000.
You will also want to review your foundation. Everything between you and the floor matters. Many mattress issues originate from foundations that are less than adequate, or mismatched to the mattress. And finally, your pillow. I can not stress enough how the right, or wrong pillow can affect your sleep. I often opine the pillow is the mattress for your head. It provides support, comfort and a quality cover to make it all happen, just as your mattress.
Best,
Maverick