I have IC, ME/CFS, and POTS too! You're right, most IC forums/groups only talk about IC and don't seem to address other related illnesses.
Weird question: Are you taking any kind of birth control?
My IC was HORRIBLE. Constant agonizing bladder pain all the time. And it's an agitating kind of pain. With POTS headaches and fibro pain, I can lie down and do nothing and that often calms things down, or at least makes it more bearable. With an IC flare, I can't settle. There's that constant urge to go to the bathroom, even if you just went, that makes it difficult to relax at all.
A few years ago, we made an accidental discovery that improved the IC dramatically: my bladder hates birth control! My ME/CFS doctor suggested that I go off birth control to see if that was making me more fatigued. It had no impact at all on my fatigue but it made a HUGE difference in my IC pain. Now I still have IC, but it's much more manageable. I've tried every possible kind of birth control (pills, patches, IUDs both with and without hormones, etc.) and all of them immediately provoke the worst IC flares. If you are on any form of birth control, I highly recommend going off it for a month or so and seeing if that gives you any IC relief.
Also, do you use heating pads? You mentioned that it's hard to rest when you constantly have to get up to go to the bathroom and I so relate. I'm pretty much chained to my heating pad, especially during a flare. At night I wrap it between my legs and over my pelvis every night and that dulls the constant urge to pee so I can get to sleep. Hot baths sometimes help too.
Other things that have helped me: IC diet, pyridium, condom pops, bladder instillations, seat cushions with holes in the middle, pelvic floor physical therapy. You said you've already tried everything though, so I'm guessing those aren't new. I'm currently on week 3 of a 12-week PTNS therapy for my IC - hopefully it helps!