[mordae]

Multiuser support is free, face detection is cheap enough embedded camera chips do it, metadata indexing is cheap with any recent DB engine, search is cheap with them as well.

The only computationally expensive thing here is face matching. Photoprism recommends two cores and 3G RAM and claims to be able to run on a NAS.

[Mashimo]

From other comments in this thread it seems you can run immich on a raspberry pi.

Any who, I don't consider it a simple sync service.

[pathartl]

It's not just face detection. It does some level of object detection as well.