I'm considering a migration from Synology Photos to this, is this supported in some way or I should transfer / reimport everything manually?

I've migrated from Synology Photos. It's pretty seamless, since Immich now supports External Library. I use Docker Compose in Synology, so basically all I have to do is just mount existing Synology Photos folders to Immich. Works fine, no issue so far.

However, I'm back to Synology Photos. I'm using Immich iOS apps. The upload/syncing is noticeably a lot slower than Synology. Gave it a few months, but it's not getting any better. Moved back to Synology Photos for now.