Minimum resource usage ROM for 1 app

Hi everyone. Does anyone know a ROM or a way to clear everything out of a rom?

I have an old galaxy s3, and I would love to use it only for my PLEX player as a remote controller...
Dont even need GPS or SIM support, only WiFi and the PLEX app :-/

Hope someone has something for me :-/

Thanks in advance!
Best Regards,
Mario

Ring & alarm minimum volume

Anyone else feel the ring and alarm volume doesn't go low enough before muting?

IMO there is to large a jump from mute to the next step.

Exynox non-rooted.

Any way to change minimum brightness value?

The screen is just too bright at a dark room with the lowest setting and without using auto/adaptive brightness. I don't want to use any screen filter apps if possible. I tried changing the value with gravity box to 1 but it doesn't seems to do anything. I am on Lollipop 5.1 stock fw rooted. Thanks