Yeah, I've come back around to this: you do not want "end users" writing Groovy, because the tooling around it is horrible.
You'll have to explain the weird CPS transformations, you'll probably end up reading the Jenkins plugins' code, and there's nothing fun down this path.
We build Docker images mostly so ymmv.
I have a "port to github actions" ticket in the backlog but I think we're not going to go down that road now.