I've never tried this so I don't know if it would work:
1. Setup the XS Max as a new iPhone.
2. Install beta profile and update it to 12.1. beta.
3. Go into Settings and erase everything so it reboots as a new phone.
4. Setup the XS Max by restoring from a backup.
5. Install beta profile.
Like I said, no idea if that would work but it's what comes to my mind.
Edit: I would at least try to restore the 12.1 backup to it first, but yeah, it probably wouldn't work.