If an org is heavily invested in GitHub Actions and GitHub App integrations, is self-hosting GitHub enterprise the only practical option?
Buildkite has built a GitHub Actions adapter... a good first step out of the GitHub Actions supply chain attack trap.
no; gitea and foregjo both support actions-style CI/CD and both serve mostly GH-compatible APIs and have GH-style apps. it's not 100% compatible (forgejo is more compatible than gitea), but many of your workflows might "just work" without even renaming the .github folder.