Priest is partly a curse class, with it’s incompatibility with necro items. It just happens that GK drops favor priest items. If there were three level 70 dungeons we might be singing a very different tune.
The priest rez takes communication to keep up and if the tank goes down, you might not have the time to wait for the ability to be consumed, the party should probably rez him ASAP. The player has a lot less control over it compared the other resurrection abilities. The priest should get a buff to show who has his/her tether buff. I’m still in the camp that tether and the side bar single target rez are not an apple to apple comparison due to the lack of control.
The strongest part of the priest heals come from the mods that replenish the players armor. It’s such a game changer and if you actually want to help other classes heals, I’d say add more armor replenish mods. Maybe retune some ‘replenish owners armor’ into groups armor. This would actually neuter UA/psychs ability to solo strong things.
I don’t really play a priest beyond leveling it but I think the most fun ability is triage. Target free-healing options are good in my book.
To mentalism:
I have done suggestions asking for mentalism’s waves to all not share the same cool down. I have found that to be a little limiting, and this change would absolutely make mentalism more fun. It is not directly a healing boost but it is a sweet support buff. The middle ground option would be make the wave that the caster used have the regular 20 second cool down, but the other waves have a 5 second cool down. The pure-wave mentalist would be forfeiting so much damage /slots that I think it would be balanced without tweaking the mods. Another wild option would be make a basic attack reduce the wave re-use timer by 5 seconds. This would then put the min/maxer in the position of eating 5 slots to be the king of the waves (surfs up).
edited to add: the best healer combo imo is pig +priest. It just feels a lot safer than the other combinations. Before my friends started that combo, the go-to healer was mental/priest, but the pig really brings a lot more to the table. I dont play pig enough to really spell out why but it's a very nice healer.