Hello reetrocker,
We have the same computer and I've been through the same thing.
The quick solution is to install an old version of the BIOS so that it allows you to lower the voltages again.
I currently have the 308 and it works OK.
309,310,313 DOES NOT WORK.
Then you should disable the automatic updates of windows, and delete the file of the current version already downloaded from your pc,
which is saved since otherwise it will be reinstalled at some point in:
C: \ windows \ system32 \ DriverStore \ Filerepository
Ex: Search fx504
I have version 308 on a USB which I install right away if the bios is updated.
Once you fix that, I recommend using "ThrottleStop".
https://www.reddit.com/r/laptops/comments/ig0zlx/unable_to_undervolt_my_asus_tuf/Greetings