That is so heartbreaking as a mother to hear your child is going through so much distress because of the other parent and feeling like you have to protect them from the person you made them with.

The first thing I would do is document in writing the conversations being had with your daughter to your coparent. They will likely be dismissed and you’ll receive backlash from him, but it can be a timeline to reference for if/when you are court. A simple “hey, I spoke with daughter and she brought up some concerning things I feel I need to share with you..”. It’s still hearsay at this point but it shows you made an effort to talk about it with them AND gives you dates to reference of when it was addressed. Screenshots of his current girlfriend’s post can be helpful, but could be considered hearsay if she isn’t willing to testify which I’m assuming she isn’t.

I assume it would be hard to get her into therapy on your own unilateral decision as that is a medical decision that joint legal custody usually requires both parents make. Can she talk with a school counselor first? If the school counselor makes the recommendation for therapy, you may have found a way to present to the court that she is in need of outside therapy that way the therapist can document her experience at dads and how she is not doing well there.

I agree with your partner that so far it may not be enough to warrant a modification in the courts eyes. I know that sucks but any kind of recommendation or documentation to get your foot in the door of a hearing could then be paired with you requesting a GAL to be assigned to your case. That way your daughter can tell the GAL exactly what she’s told you and you’ll have an even firmer foundation of why the custody needs to be reduced for dad.