If you're coming off a Pi, you're going to be getting emulation lag anyway. But all the same, I run a GertVGA666 for native RGB straight off the Pi (out of GPIO rather than through the HDMI interface - 6bit per channel only, but fine for emulation of old consoles). If you want analogue 8 bit RGBS, you can get something like a RetroTink (the first model, which is a RPi hat, rather than the RetroTink-2X which is a standalone ADC). All much of a muchness. Regardless of any of those solutions, you're going to get lag from the emulator and low speed of the RPi.