My team is set up with fist team line up, few subs, and investments for the rest, young trainers with quality too low for most matches.

I will identify a skill or 2 that I would like to improve in a young first string player (young and expected to play important first string role for more seasons) and set up the drill regiment just for him. Then I add young investment characters to train with him to fill the training bonuses.
I do this instead of running my main 11 through many drills that don't specifically improve their individual balance. But I also assume individual skills can make an impact, which is unproven.