single training

my suggestion:
I'd like to be able to train single players, and not my whole team

You can already do it. Set the whole team to training level 1 and then click on the green vertical bar (left to morale and condition) of a single player and choose training level 2 (normal) or 3 (hard training).

That way you can train specific players and give others a break.

Btw:
Take care with hard training. It is more injury risky.