I leveled Diona first because I had more of the mats for her.
She's cool but my main DPS is Hu Tao....who needs to be below 50% HP so Diona's healing isn't good.
Same goes for Bennett....Hu Tao has to be at literally 5% HP at the start of a fight because his ult heals her too fast. The attack buff he gives isn't even worth it for her.
Gonna level Zhongli next and just run a shield on her. Diona is good for pretty much any other DPS, though.
Fred.
Hu Tao's damage output is insane but you basically summed up why I prefer Diluc over her.
Zhongli solves it completely though because of the strong shield and no health regen. I actually stopped using Diona once I got him.