Whew! Sorry @dwhdai! It has been a while - but this is such a good question, I couldn't resist!
I think a GitHub Action for writing the rsconnect::writeManifest() is probably not feasible because it would require installing all of the packages / etc. in order to generate the manifest! Are you managing packages in a .lock file of some type already for reproducibility? renv.lock or something similar?
What might be a more realistic scenario is a "git hook" for automatically running rsconnect::writeManifest() in your dev environment as a pre-commit or post-commit hook, something like that. Does that seem like it might be feasible in your environment?
Would definitely love to get more info on the workflow you are using today, as well as your ideal workflow! And if you have solved the problem in these past two months, I would love to hear how you did it! 