I think referees and VARs have to create a comprehensive document explaining rules, situations and when to do what. And this needs to be followed consistently across Europe. Also, it needs to be made clear whether the decision was correct in the first place or not a "clear and obvious error".
Think Martinez handball vs Sociedad, we were told clearly that if in a block, the ball ricochets of the body to hit the arm, its not a handball. Did they believe it wasn't a clear and obvious error or were they convinced that's a handball? Why the referee persisted with that decision needs to be made clear. Similarly, my understanding of the Casemiro situation was that if a player has studs up but gets the ball, it usually is a yellow. I can understand if that was given as a red and they didn't want to change it. But in this instance, they decided to re-referee the incident.
Somewhere, the application of the law needs to be made clear so we know whether they got it right or not and what led to this thinking. It is easy for us to cry foul because we remember those instances where we were wronged than the other ones. But the principle remains that without transparency, there is bound to be frustration and the refs would always be at the receiving end of it.