https://retropie.org.uk/forum/topic/30743/lr-duckstation-experimental-new-playstation-1-emulator
options for removing dithering, increasing internal res etc are in https://retropie.org.uk/docs/RetroArch-Core-Options/

you can also do something similar in pcsx_rearmed via
https://retropie.org.uk/docs/Playstation-1/#video-double-internal-resolution
and
https://retropie.org.uk/docs/Playstation-1/#video-disable-dithering

before implementation in pcsx-rearmed it needs the GL pcsx plugins (i believe the PEOPS plugin in pcsx-rearmed standalone is an earlier version of Pete's) working. these will be MUCH slower on a pi than the very optimised NEON software plugin that pcsx-rearmed currently uses.

so, i don't think it would be playable on a pi. you can run a backported version of this in lr-beetle, which is libretro's mednafen psx emulator, which has a PGXP implementation. it runs v slowly on a pi, also.

@peterman only works with certain open gl plugins. i think it's working in lr-beetle, but that's unplayably slow on a pi.