My first choice solution would be to get a MARRS bar which is the best bar I have ever used in many regards. An expensive and most likely impractical option.
My second choice would be get a top squat attachment for far less.
If you just work mobility that will help a lot, but use something like a Kabuki Shoulderok or similar instead of just you tubing how to increase shoulder mobility. A lot of those videos will do more harm than good.
Flossing can help as well if you can get someone to help you wrap the shoulders. If you don't have floss or want to buy it, you can cut a bike tire intertube, same thing.