Sounds like you've got some challenges -- sorry to read but also great to hear the persistence.
I do think you're on the right path, and would suggest some non-gel bibs that have a good chamois. I personally love the KISS one on Castellis, as well as Giordana and Hincapie. Not a fan of Assos despite what everyone says -- just felt odd to me. I looked it up -- looks like Castelli goes to 3xl
Size Charts | Castelli ? An Unfair Advantage
Two other things -- are you mostly on a road bike? If so, tires can help, but it sounds like you've likely tackled that based on the thoroughness of your replies so far.
Second, on riding style -- how much are you in the saddle vs. out of it? At your size, and assuming fitness is the goal, rising out of the saddle more and training shorter distances but varying intensity will do much more for you, and be more comfortable as you'll ultimately have less weight on your sit bones, both in absolute terms and in terms of %.