My understanding is that it's best to choose the academy players that have a rating of 39%, 59%, 79% (or very close to it), so that you only have to train them a few times and they'll suddenly gain a star. That way, it's possbile for players to offer to buy your player for T-bucks and in-game money. If you intend to play your 5* GK in most or all games, it might be worth training him to gain an SA because playing games will add to your stats/SA, as well as the usual training.

Last season, which was actually my first season, I started training my main ST to have AMC (also requires 50 training points, like SA). I didn't know at the time that it could take a long time. But he actually took less than a season to get the AMC position. He was the top scorer in my league, so he got the +30% finishing reward two days ago. He's still just 21 and currently on 5 stars. I'm not sure how I got lucky with this player on my team to be honest. He seems to train fast, and being top scorer obviously helped by getting the skill points reward.

I feel Nordeus could perhaps offer a "clean sheets" award for GK. It wouldn't be hard to implement - just do it like the ST. The GK with the most clean sheets in the league gets +30% in one of the GK stats. There isn't even a tangible reward for "most assists" in the league - again, Nordeus could offer maybe the playermaker SA (if the player doesn't already have it) or +30% in one of the player's stats.