So here is a rather embarrassing update. After a week of using the new phone, the google app on the infotainment screen in the truck prompted me to log in again. I guess it took THAT long for the system to realize that my phone had changed. Anyway, the text to voice tool started working again after this.
So I guess it was a permission issue. No idea why it took a week for the system to realize the phone had changed.
So, in short, there does not appear to be an Android 15 issue at all. Just an odd delay in authentication.