All my first team are 6 stars with 2 7 stars due to training and signing the right players.

Between matches, I count how long it will take to naturally heal to 99% and train accordingly.
Then when I save enough green packs I put them to good use with fast trainers.

As far as youths go. The free one is useless except for lowering quality at the end of a season and the higher one could be useful but often a waste of time if you have a squad like mine.