If you play longer, you'll see that your best players have more consistent form. Anyone who is always 7 or 8 is a keeper.
The ones you don't like are ones getting 5s and 6s.

As for tactics, I assure you they matter. I have taken down MANY powerful teams that were clearly pumping real money into the game because I knew how to counter their formation and strategies.

It really depends how many players I have that I can count on, this season I don't have a strong team because many players I bought didn't work out, and 2 of the stars I've had for 8-9 seasons are finally slowing down.

I've had plenty of 6 star players suck, and plenty of 3/4 star players be solid contributors, so there is a lot more going on than just number of stars.