The new DLC is not on gamepass. The original game still is.
I still have 3 separate games going, one is my original owned copy on Steam, one on Xbox, one on Win10. I’ll wait until I finish one before getting the DLC though. It’s a long and hard game. I still haven’t resorted to walkthroughs or hints yet, solving everything myself, which is very satisfying.
Switching between the three versions really helps. If I get stuck on one version, going to one of the other two to catch up lets me play through old puzzles again that I already know the solutions to, but playing through them triggers a connection and “aha” moments for the other ones that I’m stuck on.