I've been trying to increase the fort's soldier so I can aid the rebels by sending them soldiers but its a very slow process, the city has high control and doesnt spawn many bandits and even when it does I still need to gather almost 800 bandits which would take atleast a 2 in game year to do, is there another way to increase the fort's soldier count?

800 bandits?? Not sure what you mean.

Normally you can  drop off  1-6 after each fight. So in a day you could do up to 12+ easily and top off fort sera at around 100 soldiers in a week or 2