Healer aggro is just fine. It makes the tank and the healer have to actually play the game. You can't mindlessly spam heals and get there and the tank cant do two rotations and go make a sandwich.

Healer aggro is a one for one basis. You heal for 1k then you generate 1k of threat. Tanks build aggro the same but based on dmg x the threat modifiers on their attacks. This creates a situation where when you heal you want to make sure they are missing at least that much health because otherwise all your getting from your overheal is aggro you don't need. Keeping a tank topped off in dungeons (Coil is different and as such requires slightly different approach to healing) is a bad idea and you should be learning how much dmg they are taking from fight to fight so you can heal them as efficiently as possible. If the tank doesn't handle aoe pulls well then you are going to be dealing with mobs regardless of efficient you are.