Probably not hard. Timelinize's data sources have a standard API with just 2 methods [0], so it should be fairly trivial to implement depend on how accessible Immich is.

To clarify, you don't grant access to Google Photos, you just do the Takeout from https://takeout.google.com to download your data first.

[0]: https://pkg.go.dev/github.com/timelinize/timelinize@v0.0.23/...