Best/quickest way to fix shoulder mobility issues

since starting yoga, I've realised that my shoulder mobility is absolutely terrible. I also assume this is probably impacting on things such as overhead position in overhead press movements and gymnastic elements such as handstands.



When I do the position below, my arms are at a 45 degree angle instead of going straight upwards. I have the same issues in seated and standing positions and find when I try to force the arms back a bit that I start to compensate by arching my lower back.







Anyway, would appreciate any input on good ways to start dealing with these issues. I'm happy enough to do relaxed stretches at home every day and/or isometrics during or after my workouts on gym days.



Thanks.