@signtoo Here is a Pi5 version of RGBPi OS4 https://github.com/forkymcforkface/RGBPi-Bookworm

Besides the pictures and the seller's page, there's no much info about VGA666RGBs. They say it's compatible with Raspbian/RetroPie/etc., so I assume it works similarly to other GPIO analog video add-ons, using the standard vga666 'driver'.

For RGP-Pi, I remember it outputs RGB via Scart, so it might not be the first choice if your monitor has no Scart inputs. Not sure about the driver, though.

RetroPie doesn't have a Doom Legacy package - it's probably something extra included. I don't find any information about the RGB-Pi OS being based on RetroPie (though the documentation has some info that include RetroPie specific paths), nor a source repository for the image - maybe ask the image author about the issue ?

@Efriim It is possible if you feed it some outlandish timings. I am not saying the ones you have posted will do that, they may well work OK, but not knowing the source and what they were intended for it's hard to say. I'm talking about the hdmi_timings ones, these can be dangerous. The config.txt is good, but I would skip the ones from your post starting with hdmi_cvt.

I'm bit busy now but will post my confirmed-working settings (for a CRT TV though) later. In any case OP should try the arcade forge image first, even if just to copy the configs from it to the official Retropie.

EDIT: my old config.tx. You can also try the "AF" and "mohl" settings (just make sure to remove #AF for example)

gpu_mem_256=128 gpu_mem_512=256 gpu_mem_1024=256 disable_overscan=1 disable_audio_dither=1 dtparam=audio=on dtoverlay=vga666 enable_dpi_lcd=1 display_default_lcd=1 dpi_group=2 dpi_mode=87 hdmi_timings=320 1 15 20 49 240 1 6 8 10 0 0 0 60 0 6400000 1 #c64hdmi_timings=336 1 10 18 46 288 1 5 6 6 0 0 0 50 0 6400000 1 #AF hdmi_timings=320 1 16 30 34 240 1 2 3 22 0 0 0 60 0 6400000 1 #mohl hdmi_timings=320 1 10 20 54 240 1 6 8 10 0 0 0 60 0 6400000 1

This should give you a stable picture, I have used these settings on many CRT TV sets, they should work on a monitor too I think (or at least won't damage it).
problem is, this will only work for few emus in 1:1 scale, since they often used different resolutions. To handle this you need to use the high-res trick (read the thread I linked to)

For that you need to change the runcommand and then also adjust the settings in retroarch.cfg (custome aspect ratio and resolutions, or use integer ON/Off option). Unfortunately I'm not near my SCART TV sets and can't remember all these settings exactly so can't help you here. You either use the Arcade Forge image, figure it out from the resources here or post on reddit in /r crtgaming sub.

My runcommand-onstart

fbset -depth 8 && fbset -depth 16 -xres 1920 -yres 240

My runcommand-onend

fbset -depth 8 && fbset -depth 16 -xres 320 -yres 240