Accepting friendly matches again, need to get my youth teams skills up! Send your invites to Reading.

Are friendlies helping with skills more than having 100% stamina?

As I learnt, the players will not improve anymore when playing matches.

That’s right, friendlies do not help players improve, they are now merely a way to play against other managers for fun or using the match to test something in your formation or tactics. Players will only improve by training (early in the career) or playing official matches (initially only training will do it). The “Next Step” coach report will tell you more specific requirements associated to playing matches, if any.