I agree, even if they make it so applying the bandage takes twice as long, this should be a good synergy with veterinarian and Controlled Riding. Horses as is, do not seem to survive very long in large scale combat.
I would even be happier if mages couldn’t heal horses on the move than the current system.
Ideally bandages and mage heals would both work while moving but at the very least make those two things consistent.
In my opinion the amount of horse sustain should somehow link to the amount of domestication skills someone has chosen. The best mount healers possibly having zero mount or domestication skills seems silly to me.