I think that playing a completely different genre has always been the best way to detach from D&D-brain.
"I wanna play Warhammer 40k!" "Or Star Wars!"
... yes I'm sure there are dnd5e homebrew hacks for playing inquisitors a la Dark Heresy or Deathwatch Marines etc., and the thought of that makes me a little queasy, but I'll stick with my hopes of getting my players that have only played dnd5e to try NOT dnd5e by going to something very different.
edit: crap you can probably just use like ... Esper Genesis or THE ALL NEW LEVEL UP A5E Voidrunners Codex to play 40k/star wars/whatever and never detach from the 5e system!