Higher level of teaching

I would love some thoughts on resources that might help my game.

I direct a small mixed a cappella group and we spend a lot of time fixing notes in rehearsal. So much so that I don't feel I've developed some higher level skills to detect and correct unit sounds, correct timbre, and even vowel placement.

When the group is finally hitting on all cylinders I usually only have a couple weeks for this higher level work.

What resources would you suggest so that I can work on these skills more and then make the most of that time when it comes.

TIA