Tank/support in general have the longest longevity so they should be the N°1 priority if you lacking a good one.Only time I could EVER imagine UoC'ing a damage dealer is if you're woefully behind the meta and you went hard on a banner but only got 1 of the 2 you need to make a 7*
I'm hardly an expert, but I imagine you could pass all the content available in this game with almost any 7* DPS unit and an appropriate chaining friend. You ain't clearing SHIT without proper tanks, a breaker and good supports.
I agree DPS get powercreep so fast it's not worth it, unless you're one away from an amazing STMR.
I'm doing every event trial/bonus stages with double Golbez and have no trouble.