Hi there,
The best way to go with training is to pick the position you want to train for, buy 3 or so 18 year olds with P Allstar or higher potential and then buy veteran/other players to fill out the squad.
It’s widely recommend to train 1v1 forwards for at least the first season as it trains 4 skills (JS, HN, DR and IS) which then gives good elastic effect to train other skills. The elastic effect is where a high levelled skill speeds up the training of a lower rated skill. For example, I did 2 season of 1v1 forwards with my trainee and so far this season he’s had 5 OD pops in 7 weeks because his HN and DR are so much higher than his OD.
As for your question regarding youth trainers, they are only effective for training 18 and 19 year olds. If you’re wanting to train a player for the national team it’s recommended to get one for the first 2 seasons if you can afford them.
If you have any other questions feel free to message me or any other English manager regarding training, they’re a lot of knowledgeable people in the EBBL