In case you want to test it here is a model of the Falcon board which I was going to use to test my own case replacement.
https://www.thingiverse.com/thing:6422444

Unfortunately I lost the openscad code when upgrading my Mac...

I've attached a screenshot of what I see on Joyride once it starts to load the 2nd part. This is on my 600XL PAL with RespeQT 5.3 and 5.4 rc1 under Ubuntu Jammy (edit: Also I just installed 5.4 rc3 from this thread with the same results). I put in the real GTIA and Pokey to be sure it wasn't my faul...