During casual surfing I found this article from Brain Baker explaining their revised backup strategy.
The most interesting part was the brief mention of PhotoSync used to sync photos to their Linux system.

This came just at the right moment, as I was exploring options how to get photos from an iPhone to a Linux machine as seemlessly as possible.
Previous attempts with ifuse/libimobiledevice didn't work reliably, and uploading the photos to iCloud and somehow scraping/downloading them again doesn't look very future-proof either.
Thus very happy about PhotoSync which runs on the iPhone and talks a plethora of protocols on the other side (in my case I opted for SFTP).

It is a freemium app with paid in-app purchase for the pro/premium features (raw photo sync and autosync in the background).
Based on the good reviews and initial functionality testing of the free version I decided to fo for the one-time purchase to unlock the premium features.

I did setup a chrooted SFTP user to receive the photos on the Linux machine.
Then configured this SFTP access in the app which worked seamless.
So far the manual sync of existing photos and videos worked reliably over WiFi.
And I'm looking forward for the geofence-triggered autosync to run 🤞🏻