PSA: Downgrade player Agreed Playing Time every season to increase squad happiness
Every year I will right click on every player and drop down one level of Agreed Playing Time ie. from Important Player to Regular Starter or Squad Rotation to Impact Sub etc.
The game will tell you if the player didn’t like it or not. If they don’t like it, you can revert it and it will be like nothing ever happened. You can also have a talk with the player and they actually might accept the new downgrade of expected playing time.
I usually get around 6-8 players expecting less games so they won’t come up to me mid-season wanting more playing time.
Hope that helps!