I will probably end up with a different solution
I teste FileRun in a LAMP app (after reading @scooke reply here) and it does exactly what I need without all the added bloat and unpolished UI of Nextcloud.
It sorts pictures by date taken
Has a better UI than NextCloud (personal opinion)
It is a lot faster in previewing images
Works with the Nextcloud desktop and Android client (or any other app that supports webdav)
Can organize photos in albums and collections (I am not really interested in this, but it's a nice to have)
I hope that it will officially come to Cloudron at some point.