Hey @Fred_VIE this could help… specially since you have problems with the GUI after reboot.
I hope this info is self explaining to you… just create a emty file and copy that stuff and into it also add .sh at the end of the filename. btw you dont see a good result when changing in the UI and using Terminal commands… so the powermizer may didnt show up the right result… you have decide for yourself which way fits better for your like.
I total with you here and for me its works after a restart, but this can be solved… 
With this little script you can also reload nvidia settings after restart:
#!/bin/bash
nvidia-settings -l
Try if this works…
You can also create a autostart command if you dont want to always clicking on the bashscripts on your desktop… there are several ways to add something in autostart…
I could even deactivate g-sync on my Monitor, this was a much bigger fight 