So coming back to the actual quastion:
for houseruling funplay: What smallest possible changes would level those 3 obviously weaker mages out?
Priest is bottom tier, but not "obviously" weaker. His 2 siblings are simply better.
For the same reason I don't think the warlords are that much weaker than all the other mages. It would only become a real issue once a new war mage, with better (swarm) abilities than the bloodwave, would exist.
Anyone can read my general thoughts, but for people that feel them lower tier I would change following:
Don't limit Vet tokens to one per creature (this can apply some pressure, likewise as Kralathor)
Make the Priest channel 10 and cost the burns 2
Jokthari I would change wounded prey to also give range +1 (for her)
I don't think those mages are weak, but those are small adjustments if you want to house rule them if you are personally of the opinion in your group they are too weak.