Some players are worth keeping even as three star.

Training at the end of a season makes your Cup draw harder, so unless you are making a push to win League I think I would wait on that

Buying players in the first week can be quite costly, so I tend to replace guys throughout the rest of the season as I find worthy replacement, saves on T but yes their Q drops a star new season if you promote.

Selling on first day of new season often gets the best price for me, even though they have aged, because more people bidding the competition ups the price, and I set it at highest.