Do I have to banish Grimm Troupe to collect all charms?

[ゲ-で]

And get an achievement?

[Watermelons]

No. With the Grimm Troupe, if you fight Nightmare King Grimm, you get the final level of the Grimmchild charm. If you Banish the Troupe, then your Grimmchild charm is replaced with another one when you go visit Dirtmouth and talk to a new NPC there.

So it's a choice. The charm you can get when you banish gives you a chance to ignore damage. Or you can fight NKG and keep Grimmchild .

[bobime, dorime, demomium]

There is an achievement for the Banishment. But there is an achievement to complete the ritual as well to defeat NKG. I defeated NKG on my main play-through and then banished the Troupe on a different run to get both achievements. Regardless of what path you choose, you always fight Troupe Master Grimm.

[Akari]

Regardless if you banish Grimm or not, you still get all charms if you explore the world thoroughly. But you’ll get a different charm depending on your actions, you either get Grimm Child for completing the ritual (Killing Nightmare King Grimm) or Carefree Melody for banishing Grimm.