You could use kopia for this (but you would need to schedule cron jobs or something similar to do it).
The way this works with kopia… You configure your backups to a particular location, then in-between runs there’s a sync command you can use to copy the backup repository to other locations.
Kopia also has the ability to check a repository for bad blobs via its verify function (so you can make sure the backups stored are actually at least X% viable).
Using zerotier or tailscale (for this probably tailscale because of the multithreading) would let you all create a virtual network between the devices that lets them directly talk to each other. That would allow you to use kopia’s sync functionality with devices in their homes.