I think these band distractions will help as well.

Following the MWOD recommended methodology, always take care of the joint capsule first, then move onto the deep tissues, and the neuromuscular bits are last.  When you do your mobs, put a little more time into the left side to see if you can get them a little closer to even.

There is another shoulder capsule mob that involves laying on your back and holding a heavy weight directly above you as well.  That would be particularly relevant if you have noticed that your shoulder blade on the left wings out when you pull or go overhead.

You’ll want to do some smashing in the t-spine area as well to make sure there’s no extra restriction coming from that area.

I have a lesser version of this same problem, myself, so I am still working on a solution. I think there are 3 main reasons why we might end up with the side to side ROM differences.  One is habitual position (if you tend to lean/slouch to one side when you sit in your chair or on your couch, one shoulder might end up in a different place in the capsule, which can be corrected with the mobs above).  Two is general tightness, and we tend to be tighter on one side based on our daily habits.  Three is the ‘use it or lose it’ principal.  If you haven’t been using your full ROM on one side as often (overuse of a dominant hand at work or in one-sided sports means the non-dominant side rarely hits those ranges), you lose it and it takes a lot of work to get back to full range.

My approach: band mobs first and foremost, smashing second, and thirdly work on as many exercises as possible with dumbbells/kettlebells rather than always using a bar.  This is particularly important with overhead movements because you can cheat a little by using the bar to generate torque and push you into a better position; the individual weights give better feedback on your form. 

Hope that helps!