100% agree with what you're saying. Having no redundant backups for things like files or photos should be a no-go.
However, wouldn't it be possible to create an automated script that can copy the relevant data from an Umbrel node to the Synology NAS?
Something like this? rsync -avz --delete /path/to/umbrel/data/ synology@<NAS-IP>:/path/to/NAS/folder/
Just set it to run as frequently as you wish. Having this as an Umbrel app where we can specify the path to the server and set the backup frequency would be a nice addition to their store.
They have Syncthing which basically does what you're saying. But it's miles away from something like Synology Hyper Backup and if you already have a NAS... why not just use that instead for photo hosting / storage?