That would beat having to install something like Backblaze on every family member's machines. Cloud backup is great but it's always better to have a local (LAN) copy and then an off-site copy.
Borg is really nice, and rsync.net is of that variety of service that are always my favourites: it does one thing very well.
Also they offer a discount if you use borg or attic (possibly others) as they turn off their ZFS snapshot system and assume your software handles that.
I'm happy with the extra price for my use, but yeah it's not the best option if you have a lot of unduplicated data.
If you're interested, here's the link to the borg/attic pricing page: http://www.rsync.net/products/attic.html
I'll probably try Delimiter once they start offering service in London, as they also have some similar low cost + high storage plans.
Do you mean backup to the NAS or for backing up the NAS itself to a third location?
Either way, there's no need for an either/or approach. Just do both. I've tried multiple backup applications and many support local and remote backup options as standard. I'm using Borg and Back In Time and with Borg it's just a second cronjob with nearly identical scripts for the off-site backup over SSH. With Back In Time I was using the AWS CLI to push the backups to S3, but I found a better deal with cheaper storage. I have about 200GB data in total but want a 1TB archive available online for older backup sets.
Side note - OP's review is helpful but Borg Backup is oddly listed under 'Attic', which it was forked from years ago. Don't bother looking for Attic if you are comparing backup tools available today.
I use borgbackup to back-up my stuff locally with rclone to mirror the borg repository in the cloud (personal Google Drive in my case) and have also experimented with running borgbackup to an offsite Raspberry Pi.