my "recipe" the last seasons is :
When I buy a young fast trainer, defender, I give right away "defending wall" or "aerial" (or both, to another of course).
I never had a fail in nordgen defenders. Maybe you won't find a guy who scores every three games or he is a F-K, C-K specialist but who cares
My defenders rarely have under 7 in their games (but rarely a 9 too), but my defense is working fine.
Also those above s.a. are working for guarantee.

Also when I get a ST, I test him in one game and I give him AMR or AML position too.
Then I give "shadow striker" which is an action a winger likes to do in the games.


Believing that progress slows with each quality star gained, you might train until just before the next star (99%, for example), then switch to SA training to maximize progress.
wasted greens
@Bunzo correct