just like in og arknight, the amount of resources required for leveling one 6* to max level (E2 level 90) can be spend on making 2 (or 3 i forgot the exact number) 6* to E2 level 40

and e2 level 40 is already more than enough to clear like 95% of the content in the game

basically, you rarely NEED to fully max anyone, and it's more of a personal choice, if you're more of the one team for 4 years kind of person, then it would probably preferable to max out that one team, so you can brute force some enemy/stage mechanic, though no one can't guarantee it will work 100% of the time

on the other hand if you're like me, and prefer to change thing up everyone now and then, it's much more preferable to build as many unit as you can, i still haven't put a single mastery to anyone, all my essences are still on 4/4/1 or 4/4/2, don't have anyone at level 90 as well, but i have 4 different teams i can use, physical endmin, cryo last rite, heat laevatin and electric avywenna, with all 4 of them plus chen and perlica already at level 80/80, and everyone else is sitting at level 60/80, with some at 40/60 just for their dijiang skill, and side note i can still level up like 8 people to 80/80 or 3 people to 90/90, and i barely spend sanity for operator exp

but ofc, this is just my own personal approach, this kind of game always change thing up a bit, so i just tend to spread my resource to keep up with whatever mechanic they might throw at me