Just an update on this, even after installing the new Flycast, I still get what I can describe as "screen tearing" or glitching when motion is happening. This isn't consistent, so maybe it's related to framerate drops?

I am using a Pi4 8GB without any mods, just stock settings for performance.

Probably the wrong versions of the chds or roms.
But anyhow: i dont think that the chd titles will run fast enough to be playable, they need much power which the Raspberry does not offer.