Ohhhhhhhhhh, nice sir, nice.
Someones got the "technology."
I'd recommend these or a high frequency and volume of chin ups, supine and prone grips.
something like a little 3x3 with a warm up set of max body weight reps. I'd use about your 4 or 5 rep max for the 3x3 and take a bit of weight off if the last sets are slow. you want hard vertical pulls with full ROM.
Pop that vertical pull work in at the beginning of each session right after your warm up. It wont likely 'wear you out' or fatigue you too much for your other training. Try it for 4 weeks or maybe like 15 training sessions. Drop your usual overhead bilateral lifts for a bit while you do this. They should rocket back up after you start up again. The single arm lifts will really help teach you to recruit motor units and push hard. with the pulling it should bust the plateau.
be sure to keep your eyes looking straight forward or slightly down when pulling. It will kewep you vertical and really work the agonist muscles of overhead press.