Thanks @jssjr and @pavel for your help and patience; that’s great news. Sounds like one of those issues rather are pretty frustating to uncover, but satisfiying when you do. 
I’ve just triggered a deploy:
and it failed to set up flyctl:
Run superfly/flyctl-actions/setup-flyctl@1.5
Error: Request timeout: /app/flyctl_releases/linux/amd64/latest
But a rerun saw it succeed and deploy successfully.
I’ll treat that as unrelated network weirdness. 