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