While this is a great approach for any business hosting mission critical or user facing ressources, it is WAY overkill for a basic selfhosted setup involving family and friends.
For this to make sense, you need to have access to 3 different physical locations with their own ISPs or rent 3 different VPS.
Assuming one would use only 1 data drive + an equal parity drive, now we're talking about 6 drives with the total usable capacity of one. If one decides to use fewer drives and link your nodes to one or two data drives (remotely), I/O and latency becomes an issue and you effectively introduced more points of failure than before.
Not even talking about the massive increase in initial and running costs as well as administrive headaches, this isn't worth it for basically anyone.
That'd be the gold standard. Unfortunately, the external network utilizes infrastructure that doesn't support specifying firewall rules on the existing separate VLAN, so all rules would have to be applied on the Pi itself or on yet another device between, which is something I'd like to avoid. Great general advice, though!