By skill work I mean working on your shoulder positioning and technique.
When you have your shoulder in the proper position and have proper movement technique/mechanics your shoulder is in a stable position.
It does not sound like doing “the classic stuff like external and internal rotations and dumbbell rehab stuff” is helping or addressing the cause of the issue.
Are you following the same training that you were in 8th grade or have you made changes to your training schedule?
Do you see any improvements with the tendonitis?
What are you doing to address this?