Brigands are faster and have 4/8 support cost per unit compared to 7/8 for trebizond archers. I think you should start replacing trebizond archers with brigands. While trebizond archers are better at attacking flanks after they run out of armors, brigands are less likely to lose units when an enemy manages to get close enough to attack them on melee, which negates their lower morale. Their better armor also means better defense against arrow fire.

Bulgaria, I think, has a bonus to brigands. Of course, brigands aren't that different from trebizond archers to warrant getting a master bowyer unless you have lots of money. Even then, I'd stick with arbalests.