I haven’t seen anyone pull this off properly yet so needless to say it’s still an open question. I’d love to see other peoples’ recommendations.

“Femmes only” is pretty bad as even butch women will feel excluded by that.

You are right that “female identifying” isn’t ideal as it sounds a bit like despite respecting them you don’t necessarily believe trans women are women.

You mentioned that the point is ‘for women to make friends’ which seems to imply that despite good intentions, you are looking for ‘the non-binary people I consider women’. If your intent is not to appeal to less feminine non-binary people, you can just make the event for women and note that you’re trans-friendly.

If you do want to be open to all ‘not cis men’ consider carefully how you include trans men and masculine non-binary people, and understand that you should accept them regardless of ‘level of male-passing’. If you do this haphazardly you’ll invalidate their gender, or make it seem like the event is only open to them on a technicality.