3PO gear after a certain point isn't really important at all except as a requirement for Journey characters. I would say the biggest advantage to having him at g12+ is to be able to put faster mods on him (I tend to slice my fastest ones to 6*, meaning my 5* mods aren't as fast). Threepio often gains stealth and protection up which keeps him alive, and obviously he doesn't need damage. I think you can wait until he's 7* to put more gear on him.
As for "finishing" a character, I never thought it was irrefutable advice even when the cap was g11. Obviously, you don't want to spread yourself thin, but in a game with limited resources, at some point you want to work on multiple teams. Like you say, now characters never really get finished since they keep adding relic levels. I'm a big proponent of a "good enough" mentality--if a team (especially on offense) is behaving just as well as you need it to in the situations where you use it, then no sense in wasting more time and gear on them. Even many the characters you take to g13 can often rest between r0-r3 because taking them to relic 7-9 won't be worth the time and resources you put into that.
