So my S.O. just informed me she may be joining a fundy church

[HeartsCanHope]

24. I grew up in a fundy church.

I left years ago because I grew tired of women and the LGBTQIA+ community being bashed all the time. My birth family, (can't think of any way else to explain it,) is still in the church, so I don't get together with them anymore. Got tired of all the "interventions" when we got together. Painful and hurtful.
If she's supportive of any type of diversity it will get old very fast. Does the church have a website with some of their sermons on it? It might be useful to hear what the minister says. Might make it easier if you want to try to reason with her, (and you can turn that off if it upsets you. Less embarrassing than leaving a church.)
I have to agree with the others though, she won't listen until she's ready. My husband never criticized my birth family or religion, and just let me come to my decision to leave in my own time. I tried to get him to go with me to church, and he went a couple of times, but told me it wasn't for him. It was never a "it's them or me" situation, my husband is a wise and kind man. I couldn't go along with the church's and my birth family's political choices, either, after a time. My birth family also made it impossible to tolerate their behavior toward my little family, manipulative and hateful. I'm so much happier without them. Holidays can be hard emotionally, (miss seeing extended family,) but at least holidays aren't stressful or painful anymore. Having peace is worth it!
I hope you can work things out. Take care.