I've played strictly with controller and the game has spoiled me to the point that any MMOs going forward should have a controller mode if people want to use it.

I put single target attacks on R2 and AoEs on L2. Important but less used abilities on the directional keys for both R2/L2. Then I do buffs on when I hold both R2/L2 at the same time. For most classes that's all I need. I put things like sprint, mounts, and LB on the 2nd Crossbar. I can switch between cross bars pretty quickly.

I basically treat each ability like a special move on a Fighting game. R2 + Triangle for this move, L2 + Left for this move. L2 + R2 and X for this move.

It gets to a point that I can have all my abilities hidden and can still fully function with a very minimum HUD.