What I hate most about bras is that they are so expensive if you are looking for a good, supportive bra in a big size (I'm 38DDD). I have to wear bras because the bouncing of my boobs hurts otherwise when I move. I used to have a lot of problems with the straps until I bought these:

https://www.magicbodyfashion.com/en/magic-clip

They keep the straps together at your back so they can't fall down your shoulders.

Oh, by the way, fun way to turn your big boobs into a science experiment: fat really is less heavy than water, because if I'm in the bath, my boobs float.