I don't really look at stars for existing players. If they are 2 stars and they have good form & contribute to matches, then I am keeping him. My reserves atm are a mix of 3-6 stars. Form over Quality.
Personally I don't have time to Power Train. I have a lot of tokens but I don't think I wanna spend so much time training a player. Usually I just play a fast trainer every match, and add additional training after the match. I just get them to an acceptable level of quality on paper. I don't really aim to get them to 6,7 or 8 stars.
If by the 7th match of the league season, I am top of the pile, then it's enough for me. I always try to strengthen two or more positions every seasons. Some seasons I buy strikers, some seasons I don't. This season I strengthened my GK, DC & Midfield. My old GK didn't "win" me matches in the previous season, so that had to change. My Defence was the strongest last season, but why shouldn't you strengthen regardless? It's up to your own judgement.
Yes, FORM over Quality