1st you do not have to buy high % players but younger players because they train well faster.

I usually do in the morning a drill containing all 3 types ( attack,def, condition) for all players to get bonuses to 10% and then to all young players i do individual trainings based on which skills is at lowest (so they rise faster), try to start with special ability at low % at higher will be impossible to get it.

Always watch videos for rest packs + double training sessions to all players (i do 4/day) and the rest ones to rise the energy for the young players i want to grow.

In the start will be hard to have all team under23 years since they cost a lot of money.

But after season 4 you will be able to keep all the squad under23 and at the end of the season to replace the so called "old players" those to goes to 24 and their % is below 100% (at the start of the season).

Good luck!