Other than the normal things like offensive IQ, speed, passing and ball handling, is there anything that I should be looking at to increase my assists and offensive efficiency?

In the flex offense, I feel like I may have made a mistake prioritizing perimeter shooting over passing.

Offensive efficiency is what you want. Assists come along for the ride, basically window dressing. You do want to have an excellent passing pg (in any set) and the better the passing rating of the players on the floor, generally the better your efficiency is going to be, all else equal. But PG is the only spot where it’s a true priority, where it’s so impactful I will give up defense and/or scoring to get more of it (in flex/motion/triangle anyway).

It’s ok to have good scoring at pg, but you really want your perimeter scoring to come from sg/sf in flex. Even PF if you can find them. Flex works best when the pg is like the 3rd-4th option, IMO.