Should I do DLC before or after ending the game?

[Athenex]

as the title says

[Darkskinned King]

doesnt really affect the game in any way if you do it before or after the ending

[SMOOTHIE_KING]

Godmaster stuff obviously is most enjoyed when you've already done everything else, but doing Grimm as you go is fine. There isn't really a wrong way to play the game, just be careful with banishment.

[Boksha]

Starting the Grimm Troupe content early is perfectly ok, but I'd save the last boss of that arc for after finishing the game, since it's considered harder than the main game's final boss.
Godmaster feels very strictly intended as post game content.
All the other DLC blends seamlessly into the game anyway.

[TyresTyco]

I would make grimm at mid game.. so you can enjoy the charm you get to the fullest.

[NouH]

About the Grimm DLC.

I would recommend you to do it AFTER you finished the main game (And have beaten the Radiance), if you want to start it earlier, then you can defeat the Troupe Master Grimm and go finish the main game, if you try to do the NKG before finishing the main game you will notice a extremely huge difficulty spike that is completely different from the bosses you are used to.

While in normal/dream bosses it just takes you 1~3 tries to defeat them and less than 15 minutes, in NKG you're gonna do more than 50 tries and spend more than 2 hours of trying and maybe still don't manage to beat him, so i would suggest to beat the Main Game first to get used to the difficulty and then try Nightmare King Grimm.