I did this. I kept both.

I have Synology photos and immich.

Synology photo is faster to load photos when you scroll to a random date. Also on mobile, the map view is fast and reliable. Also the Android widget can show images of a specific person instead of a random photo.

Immich has better face detection and significantly better search tool. But is super laggy when you're looking for a specific date and time and location.

My Synology is a DS 1821+ with dual nvme cache and 32gb mem My immich is running on a proxmox using a late model Xeon processor and NVME gen 3 and 32gb mem. Nothing else is running on the proxmox.