If I understood correctly, it sounds like you could probably change very little of your current setup and just add in an extra step in a script where you use their CLI to upload new content after you do iOS backups [0]

Files get hashed before upload to avoid duplication, so I would imagine that this would also be fine to use in tandem with the mobile app; the app pushes stuff out as it's designed to do, whenever it does it, but then if you upload from a manual backup as well, it would only ingest stuff that's not already in the archive.

[0] https://immich.app/docs/features/command-line-interface/#qui...