Everyone will still need to use Xcode for at least some debugging, no way around that.

As for the builds, your agent probably already knows how to do a lot of this from the command line, although explicit suggestions can help it build faster for different situations.

As for XcodeGen, you may find it unnecessary overhead if you're already using Xcode file system synced groups.

For iOS my biggest suggestion would be to enable App Store Connect skills for your agent (https://github.com/rorkai/App-Store-Connect-CLI).

With this not only do you not need Xcode all the time, you also don't even need to be near a MacBook.

Just make changes via Codex on your iPhone, the tell asc to build and upload to TestFlight, download and run the new version, iterate.