Well, for your problems during rehersal, I would suggest to buy a cheap digital console, we use a cheap Beringer XR18. It has 6 busses, so everyone can dial in his/her monitor sound to their liking. Of course these have many possibilities to tweak (or „mistweak“), but if you run pretty basic, it should work - a good singer won‘t sound bad, even with a flat eq.
But this would require to reherse with headphone, as we do it.
In fact, at rehearsal, it's not the most important, we are in circle and can hear each other. We already have the equipment...
It was an example to show that even the music room's manager doesn't know how to tweak a mix table....