I've had a lot of issues trying to get my nvidia settings to stuck at reboot. This is because the display settings from budgie overwrite the settings from nvidia. i don't know if this will help with other settings but it will help you to not have to rearrange your displays every time you reboot your system.
Its 4 simple steps.
1. arrange your displays in nvidia settings , i couldn't find mirroring in budgie display settings so i had to go this way.
2. open budgie display settings. they should match your nvidia settings at the moment but it wont stick after reboot.
3. change the refresh rate of 1 screen to a different value like 59 Hz. Apply settings, keep configuration.
4. change the Hz back to 60 Hz (or your refresh rate). Apply , keep configuration
This simple yet working fix helped me get rid of one of my most frustrating issue's with any linux distro, i have tried ALLOT of diffferent things like nvidia-settings --load-config-only at boot, adding 20-nvidia.conf to /etc/X11/xorg.conf.d/20.nvidia.conf ,... this is the only thing that works.
Hope this helps some of you