Might consider more focus on DB or single handled grip machine work, could help a bit. Good Luck.
Several years ago I had a minor size discrepancy between left and right pec.
More significant was a dramatic strength imbalance between R & L arms during pressing.
Switching to strictly db and unilateral machine work corrected about 95% of the strength issue, and the minor size difference is now practically non-existent.
The majority of my training is done like this now, and has been for about the last 2 years.
Would definitely recommend trying for at least 3-4 months.