Hi CEMDaughter.
Welcome to our Mattress Forum 
all the way over from Reddit! We’re glad you found us.
Thanks for sharing a bit about your sleeping positions, health history, and budgetary restrictions. Sorry to hear you lost your job due to Covid. I understand that you’re also unable to test in person, so I will do my very best to help guide you.
As you’ve already figured out, adding a topper as a sort of “band-aid” to fix a sagging mattress, unfortunately, never helps remedy the issue and it is a sure recipe for spinal misalignment and backpains. To add “insult to the injury,” being a stomach sleeper, you are very likely exacerbating any lower back pain as this is the worst position to sleep in as it causes sinking down too far in the pelvic area into a “swayback” position. For your stomach sleeping position, you may wish to review the guidelines in this [url=Your sleeping style, preferences, and statistics - Your sleep positions - The Mattress Underground] Sleep Positions Article [url] so that you avoid hyperextension in a swayback position that can cause back issues.
Have you had a chance to test it before? If so, what did you base your decision on? Did you get the 10" or the 12"?
We do generally recommend that people stay away from big name brands and mattresses from big box stores, due to the fact that they very rarely meet our Mattress Durability Guidelines as it pertains to most people. That said, sometimes these mattresses work just fine for some people that look at lower budget options! Given your rather low BMI, I don’t see this being a major factor or issue however your severe health concerns and chronic pains might present some challenges in terms of ensuring that you have optimal comfort and support over a long period of time.
Allswell Luxe has 2" of copper infused memory foam in the comfort layer, followed by a 1" transitional layer of high-density foam of unknown densities, For your BMI, I’d generally recommend a minimum guideline of 4 lb per cubic foot density for memory foam or 1.5 lb per cubic foot density for polyfoam as a reasonable “cost/durability compromise” If you manage to find the densities please list them here and I’d be happy to comment upon it. In general, we recommend NOT basing your decisions on reviews because each individual has specific needs, and preferences and a different Mattress buying personal value equation that is most important to them. however …it looks like this Redditor offered a few lower budget recommendations that may work for your lower budget.
The Sleep EZ Roma is a quality mattress and at $700 you won’t be able to beat the price too easily. It’s 100% latex and, as a vetted Trusted Member of this site, meets our rigorous quality/value, customer service criteria and in this case, our durability guidelines as well. While this product is $200 over your budget this may be well spent on your health having restful nights by getting good alignment and support.
Latex is similar to memory foam in its ability to respond to pressure in a very local area, for a cradle and offer good pressure point relief. While memory foam has real strengths in the area of pressure relief and movement isolation, it also has some drawbacks because of its very different qualities and structure. It tends to sleep hot for those who are sensitive to this and even the newer generation more breathable memory foams are not as breathable or open-celled as other types of foam. It has little resilience, is a poorly supportive material, and it is difficult to control how far you sink into a memory foam layer as it may soften over the course of the night and what started out as correct spinal alignment could turn into a “hammock” position and a backache by morning. So, I’d make sure before purchasing a memory foam product that you are OK with these memory foam “traits”.
At a quick glance, some mattresses worth considering from our Trusted Members near/within your budget would be…
[indent]The Ghostbed
The Noelle or Emily from Quality Sleep Shop
As well as quite a number of mattresses from Mattress To Go[/indent]
You can also browse the sales/discounts available to TMU subscribers like yourself at the Trusted Member Directory linked here.
If you can find and share the layer specifications on the other mattresses you mentioned I’d be happy to offer commentary on how those may match up based on your BMI and sleeping position, but it’s possible with the resources now at your disposal that you’re able to do this evaluation on your own! I’d make sure to look at the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones which would include the major brands such as Serta or any mattress where you aren’t able to find out the quality of the materials inside it.
But, more importantly, any one of our Trusted Members will be able to expertly guide you towards your best fit from their selection. Because of the current circumstances with Covid, I cannot emphasize enough the importance of having a detailed phone conversation or chat with the retailer/manufacturer you are considering purchasing from. I’d make sure you share your sleeping statistics, health concerns, budget, and history with them.
I hope I have given you some useful information to help you decide how to move forward. Please do continue to check in with updates.
Phoenix