Actually big response!
Coaches in the game have a wide array of approach to the game : casual, regular, invested and hardcore. Of course you get the best training by getting harcore coaches on board BUT those are much more likely to train NT players, MVP+ guys, and are therefore of little use to you.
You're right about the types of managers and the little use for our U21. In my personal perspective, I'm also training a big NT prospect. Not planned, but I had an excellent 18r old from the draft. After he's totally ready for NT, I will train U21 players just to help our U21 and boost the quality.
For U21 prospects, I can name few managers who are training the players as planned. But I don't think it's logic. Because most people want to earn quick money and they want quick results. This can happen by training U21 players. It makes no sense. But maybe these managers belong to the first group, casual coaches because this is a long-term game...
So you have to rely on regular and invested coaches to get your U21 trained properly. A solution that might be worth considering, id relying on hardcore coaches with starter teams e.g. fresh Utopia teams or newcomers that are doing very good despite a rather fresh experience.
You as staff member for NT also have a lot of xp with giving advice and it's also something I've seen that new coaches need to be guided and they rely on advice from others. The frustration is that they answer and try it, but drop out in a couple of seasons. Not all of course, but a lot. New Utopia teams seems to be a better idea. Like you said, these are hardcore managers and they start fresh. This can actually work and is indeed a solution.
So you're experience is probably better advice than you think :)