Hi augtistic and welcome to the Mattress Underground 
happy to have you here!
I am returning my Bear Elite Hybrid (Medium) mattress that I bought during the holidays, and I am slowly losing my mind trying to decide what to get as a replacement
Sorry to hear of the lack of support in your recently purchased mattress. You have very specific needs, and I can understand your frustration. Keep in mind any mattress you finally decide on will be unique to you, based on Stats (height, BMI, sleeping position(s) and underlying health issues) - well done providing those! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences). Since you spend more time than average on your bed, you are correct in determining you need something that is comfortable, supportive and durable. Your hyper mobility disorder and chronic pain make it even more important to find a suitable mattress.
So, starting with the basics first: you may want to read the Mattress Specifications You Need To Know and compare these to the Mattress Durability Guidelines you can see the durability of various mattress materials for your consideration.
I looked at the Bear Elite Hybrid mattress; this is a 14" mattress with a cover of pcm fabric with celliant fiber (this supposedly converts body heat into far infrared radiation & reflects it back to the body), a comfort layer of copper infused memory foam of unspecifired thickness and density, a transition layer of dynamic foam of unknown composition, thickness or density, pocket coils of unspecified thickeners, gauge or coil count, and hd base foam of unknown density. I am unfortunately not surprised this was not a good fit for you as there’s no way to assess any of the components with the limited information this manufacturer provides. You have a fairly generous budget and should be able to find a suitable mattress in your range.
Materials are more of a personal preference…for foams, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. I would avoid any mattress that lacks transparency on the thickness, density or composition, as this can hide inexpensive ‘filler’ foam of insufficient density, which can ‘wear out’ prematurely leaving ‘ridges’ and ‘valleys’ that can cause aches, pains and even alignment issues, and be problematic to sit on for long periods, but a well made foam mattress of good density and materials would provide support and durability if you prefer memory/polyfoam over latex.
I have searched the forums and r/mattress and have read tons of reviews on SLTD, but I haven’t seen anything that speaks to my annoyingly specific combination of needs.
Is there a particular mattress type or material combination that would be supportive for a heavy hypermobile body, comfortable for both back and side sleeping, flexible for use on an adjustable base, AND durable for a excessive sleeping and frequent sitting? Or am I asking for the impossible?
What blend of materials would hold up best to having body weight concentrated in the middle of the mattress when I’m using my bed in an upright, more seated position, sometimes for extended periods of time?
Since you are not able to ‘test in person’ any mattresses, it’s difficult to advise you on what type of mattress you should look at: latex is one of the most durable materials you can have in a mattress, and would supportive and breathable; but not everyone enjoys the feel of latex. Memory foam can also sleep ‘hot’ for many consumers…A hybrid mattress - (with coil gauge of less than 13 for larger sleepers) would provide additional support with latex or foam comfort layers, if, again, you enjoy the feel of pocket coils…an all latex of correct density for your Stats and PPP would provide good support and provide comfort as well. There’s simply no way for anyone to tell you what mattress will work for you - and reviews should be read with a skeptical eye as there’s no way to tell what is a real review and what is just thinly disguised marketing. You and only you are the final arbiter of comfort and support for your sleep system, so dismiss all the ‘hype’ and focus on the integrity of components while you look.
I’ve been looking at all-latex custom layer mattresses so that I could swap out layers as they age. Would I be better off with all-latex or a latex hybrid?
What thickness would be best for my weight and use on an adjustable base? My base has a lumbar support beam that doesn’t go completely down so I’m worried I’ll feel this too much with a thinner mattress
As I said above, latex is extremely durable, breathable and in your desired firmness
would be very supportive…whether all latex or a hybrid would be up to you. Thickness is also not a ‘fixed’ number for all sleepers; usually a mattress of 6" or above is recommended at minimum, but depending on the clearance of your adjustable base and keeping in mind you don’t want to ‘scale’ the side of the bed to get in and out of it - a 8"-11" thick mattress would be fine, depending on the weight capacity for your base. An innerspring support layer could add additional inches to the mattress profile. It all depends on what you are comfortable on. You can read more from @Phoenix on layers here to see how all the layers fit together into a good sleep system.
You may want to look at the offerings of the Trusted Members of the site, such as Mattress to Go who carry latex, foam & hybrid matresses; Arizona Premium Mattress and SleepEZ carries latex and latex hybrids, while GhostBed has a variety of latex, foam and hybrid mattresses as well. Any of these manufacturers, if you provide them your Stats, PPP and mattress history, can guide you to finding a solution that will meet your specific needs. These have all been vetted for quality, transparency in materials and construction, generous exchange & return policies and great customer service, and can help you cut through all the options to find a sleep system that matches your needs and lasts for many years. Just let them know about the model of adjustable base you are using to make sure they will be compatible, so you don’t inadvertently void any warranties.
I hope this gives you some direction for your search…I wish you success in finding a great new mattress; and if you see any specific mattresses you have questions on, please let us know and we can possibly comment on them and how they may work for you and your needs.
~ Basilio