Can’t Sign Out of Apple ID Without Erasing iPhone

Hi, I’m trying to switch from my partner’s Apple ID to my own on my iPhone without losing any content. I want to keep all the data on the phone, but when I try to sign out of their Apple ID, I get an error saying, “There was a problem signing out.” I know Find My iPhone needs to be turned off first, but that also gives me an error when I try. I don’t want to erase the phone, I just want to remove their Apple ID and sign in with my own. Is there a way to turn off Find My iPhone or remove the account without wiping everything? Any help would be really appreciated