It sounds like we have similar setups. I do the same with syncthing, works great, and not only backs up my photos but everything else on my phone like custom ringtones, notifications, exported backups from many different apps along with full neo-backup exports... basically all the common /sdcard/ directories like: Audio, Backups, DCIM, Downloads, Pictures, Documents, Screenshots etc.

I'm interested in immich for it's multiuser sharing so I can easily share photos with others in the house. I have a huge directory of images, all sorted in folders, so until I can add that read only, immich isn't an option for me. I tried setting it up with the monolithic docker image, and it didn't import the directory the way I wanted it to, and seemingly made full copies of all the images into it's own upload directory when I tried importing with the cli-tool. I was looking at it recently and the read only mode seems early stages. How do you like it so far?

Immich seems like it's aim is to be firstly a phone photo backup solution... and that is not what I want... I already have a backup solution. All I really want is a mobile friendly way to look at all the photos I have already. PhotoPrism works exactly how I want but the one feature it lacks that I would really like is multiuser. I have seen there is a workaround for sharing with PhotoPrism where you can run individual instances for each user and then share a common directory... and right now that is preferable to immich for me unless they sort out the read only feature.