Should I just go ahead and sell any 3* / 4* that I have evolved AND maxed affection with if they're still weaker than my strongest 20? Or is there more stuff that can be unlocked if I max a girl's level AFTER evolving her?

It feels like that should be the goal for a completionist (which I happen to be), evolve and 100% affection a girl and then sell her before moving on to the next one.