I would like the end result to include remote and encrypted backups.
I’m considering maybe a Synology NAS with an APFS partition for Time Machine and a BTRFS partition for Linux backups.
The Linux laptops might backup to the NAS with Restic.
The Synology NAS might then backup to BackBlaze or another cloud using Synology’s Hyper Backup or also Restic.
Have I missed a better plan?
Borgbackup. It is an ugly command line program. But it gets the job done and ticks all the boxes. But if you accept a recommendation try to use one of the friendly frontend like emborg or borgmatic
And you don’t need different partitions so you can save space.
For Macs: I use Time Machine backups on unraid (nothing fancy but it has built in settings to support it effortlessly). There’s also https://hub.docker.com/r/mbentley/timemachine that I tried out a year ago.
I haven’t tried over vpn but as long as you can mount the share , it should be fine.
Both Synology and qnap makes solid products. Just make sure whatever file system you pick for the NAS that it can do snapshots. Wasabi is better if you are long term archiving, but for backups that churn, BB is way cheaper. LGTM.