So you're having problems gaining back the last 10-20 degrees in overhead/somewhat externally rotated position if I'm reading what you wrote correctly.
Here's some things that may help that you're not already doing:
1. Hanging on a bar for an extended period of time (30-60s per rep) just to stretch everything out better. However, to put a twist on what you are already doing focus on activating your shoulders and holding for 5ish seconds and then letting go and relaxing everything. That should help a lot more than what you are already doing (kind of like PNFing it).
2. Doorway stretch... except facing the corner of a wall (convex corner). Much better stretch.
3. Wall slides... I also like CORNER wall slides in the above (#2) setup. These erally stretch out anterior capsule and chest/lats pretty well.
4. More external rotation stretching. Emphasis on longer and not necessarily harder.
When you are doing a lot of the stretching focus on pushing your hands as far away from the shoulder as possible... get it into that active position which will put much more stretch on the capsule and muscles which is what you want. Plus, help to not impinge anything