I think what you propose to say about it not being a good fit is fine. I'd give two weeks notice but tell them you're willing to leave earlier.

If you think they're be receptive to hearing that you came on board with the understanding of ABC, but now that you've been here a few months, you understand that what they need is actually XYZ, and thus it's not the right fit for either party - go ahead and say that.

But if they already know that they misrepresented the position (and they likely do), then you're not going to tell them anything they don't already know, so bringing that reality up isn't going to change anything, and might become a distraction from the simple, clean break you want to have. When you give feedback, it opens your decision up for questioning. If your decision is not up for discussion, then why bother? Most organizations that are dysfunctional don't want feedback and have no way of processing it when they get it.

Tell them honestly why you are leaving-bait and switch. They have already invested in you, and are going to have to invest in someone else. Give them the info that they need to change their ways and not keep wasting money on a revolving door.

Been at job for 8 weeks. It's not working out (bait and switch). Have offer coming from another company. If I accept, how much notice do I need to give? 2 weeks or can I get away with less? What's a not super offensive way to tell them I'm resigning?

My goal is to not put this on my resume as it's likely to not make a year and it's really off my career path. Networking contacts from this job would be zero.

I was thinking of saying "it's not the right fit and I want to save them from further investing in me".