It tonemaps HDR to SDR (see this thread). AFAIK, there is no way to adjust color, brightness, etc. Plex for Mac/Win uses the mpv video player (mpv.io), but does not support the use of a mpv.conf file.
You might try Plex Media Player. It is also built using MPV. It supports using a mpv.conf file. You might be able to adjust brightness that way.
See Plex Docs -> Plex Media Player -> Advanced Configuration
