Sorry to hear your situation.
While I've never experienced arthritis, I do have pretty bad scoliosis, and I thought I would never be able to bind. Instead, I've always worn tank tops underneath my t-shirts (usually women's tanks, since they're tighter and help hide my unfortunately puffy nipples). In some shirts I can get away with it, but most of the time you can end up seeing the shape of my chest. :/
I actually finally ended up buying my first binder a few weeks ago for my Otakon cosplays. I bought a GC2B half binder, and bought a medium when I actually measured for a small. I feel like a small would definitely give me a more natural looking shape, but even the medium is tough to wear through an entire day (and I certainly don't wear it every day), so a small would be out of the question. I'm kind of curious if a large would be more comfortable for me, but I also don't wanna be out extra shipping if it doesn't bind enough and I have to return it.
I'm not sure how much you're trying to hide, so I'm not really sure what might be your best option, but I have heard of some guys taping their chest down everyday in lieu of binders. You'd have to look up the best techniques for that; I'm not sure how people do it every day without damaging their skin or nipples. Other than that, tight tanks really do help. I work in a very humid kitchen every day and sweat the moment I walk in the door, so I've never tried wearing multiple tanks before. Maybe you could try it and see how it works out.
Good luck, I know it's difficult being forced to not bind. Hopefully you can find something that works for you.