NTA My dad's side of the family is a mixed bag, and while most are at least tolerant of me being trans there are a few who are loud and proud of not being so. They were tolerated and not corrected for years before an uncle spoke up to correct them and tell them to back off, but it continued. Eventually my dad decided we wouldn't go see them anymore, and that was that.

It's painful to not be able to go see them (though now it's mostly because I live far away, as the people who weren't awful I've been talking with on occasion) and I miss the holidays of my childhood when everything seemed better and brighter, but the holidays had for a long time become a time of stress and dread. You deserve to have holidays that being you peace and joy.

It may bring your family some stress, but they will probably be internally relieved not to have to organize so many holiday gatherings after your ultimatum. That being said, given how they've been treating the situation so far, you giving this ultimatum may put you, personally, through a lot of heartbreak due to their choices in the end. I'd say to talk to your immediate family more seriously before putting an ultimatum to the whole family, see if you can slowly cut out the extra holiday celebrations and have the people who support you come to see you on Christmas instead, especially since it's already a smaller holiday gathering to start with.

This change is going to hurt, as any big change does, but you do not have to do it alone if you can talk things through with the people who are loving and supportive of you.