I'm having this problem with my iPhone XS Max after my updates to 13.5 and 13.5.1
Try this:
- Boot in recovery mode and connect to your computer. Restore (to 13.5.1)
- When the phone starts up, try to get through all of the set up screens as quickly as possible, and don't let the phone display dim. No passcode, no FaceID. Do not try to restore right away.
- Once you're done with set up and you're on the main screen, go to settings display & brightness and turn off auto lock.
At least for me, these steps stabilized the phone. Now you can try logging into icloud and restoring your phone from backup.
My phone is prone to rebooting if I use messages or if the lock screen is activated. (in those cases, try to tap quickly on either the phone or flashlight icons - those seem to stop the rebooting)