I hear you on bra issues. I don't know if it's my Stomach Issues or my Wonky Cartilage Issues but I can't wear any of my old bras anymore without wondering if I'm about to simply just break in half straight across my diaphragm. There is only so much I can do in what I usually consider a "sleep bra" with minimal support.
As for the poses, when I'm not swamped with work I do a lot of pilates and used to be big into yoga. A lot of these positions are just not good for your back if you're arching up on top of your breasts. I would suggest looking for adaptations that do the same thing (like instead of child's pose there is another one where you are on hands and knees, like cow, and then you go back with your arms extended all the way in front of you, it also lengthens and relaxes your lower back).
You can find some good info on modifications if you look around, as long as you ignore the 500 results that come from "yoga to get bigger boobs" which is frankly sort of disconcerting.... for example,
https://www.yogajournal.com/practice-section/5-easy-adjustments-for-poses-that-can-be-uncomfortable-or-impossible-for-women-with-large-breasts
I imagine it would be easier to find better adaptations working with a teacher, but not sure if that is among your options or not.
A good point I see in these discussions, and I wish I had thought about this years back as I was trying to breathe while being smothered by my own boobs doing handstands, is that yoga was invented by men, for men's bodies. I think it's easy to get into feelings of good and bad and ability and non-ability (i'm hyperflexible and I get a lot of kudos from teachers. it has nothing to do with my ability. a smart teacher will see right through me and make me do adapted moves with slight flexion, which are about 99% harder), and I wish that weren't the case.