I tend to do manual bootstrapping with local execution as well but when using Forgejo/Gitea and corresponding actions you could also init the repo, start building your pipeline and use act to run it (locally at first if course) and as soon as you have your infrastructure in place you could continue to use the same pipeline there?
Still not ideal/perfect but if you don’t want to depend on some SaaS then this at least already runs the automation as it will be later?
If you don’t mind I’d be interested in how the cloudinit for proxmox looks like 😁 So far I only used it for Azure and Hetzner Cloud to bootstrap Coder machines but from your description I’d expect that should work. Very cool approach!