You can back up to Immich using various methods, including dumb file copy into a dropbox folder. For a while, I was using PhotoSync that uploaded photos to my NAS with Immich using WebDAV.

Immich also has an app that can upload photos to your server automatically. You can store them there indefinitely. There are galleries, timelines, maps for geotagged photos, etc.

The app also allows you to browse your galleries from your phone, without downloading full-resolution pictures. It's wickedly fast, especially in your home network.

> Does it preserve all this additional data Android cameras add, like HDR, video fragments before photos, does it handle photospheres well, etc?

It preserves the information from sidecar files and the original RAW files. The RAW processing is a bit limited right now, and it doesn't support HDR properly. However, the information is not lost and once they polish the HDR support, you'll just need to regenerate the miniatures.