The only solution i think is to make significant cuts in player salary/formula. Instead of adding revenue in other aspects of the game. Its a simpler fix. The cost of making a NT player from age 18/19 does not reflect current market rates. It hasnt for quite some time. In fact it is cheaper to acquire one in the TL. Prices of top tier rookies have also gone down quite a bit over the BB decades time line this i believe is a reflection of the economy. If the significant cuts are made in the salary formula they will naturally have more value to more managers trying to compete. It will also incentivize trainers to actually train because top tier talent will be too expensive to buy.
To be perfectly honest if my intent is to profit take i would most likely sell my trainees at or a few weeks before they turn 21. I can recover my cost and earn a few weeks of monthly revenue. Thats just my take. Again not everyone in this game likes training. In fact every season our own NT struggles to find enough trainers. I am sure other NT/U21 coaches also face similar challenges.