Thats what I did as well.
Spent hours trying to resize the partition, couldnt get it working.
In the end, it was just easier to fresh install and use migration assistant.
Finally got a functional Macinabox Catalina!
Ryzen 1600AF with 4 cores + 8gb ram assigned
Catalina had no problem detecting my GPU (MSI Gt 710 1GB) Native support, no web drives needed.
Awesome work SpaceInvader !!