I know about those. Since the whole point of innernet is making wireguard configuration simpler, I figured maybe integrating something like that is worth doing.
At vultr you have the option to "bring your own ip", as they call it. This really just allows you to use BGP to announce whatever prefixes you own. You could have a /40 behind each of your servers if you wanted to.
If you are running postfix as your mail server, you could use something like [0] (or write your own solution, utilizing a database or whatever, of course).
I'd use zfs (consider freenas as your os, i like it) with raid-z3 (triple-parity, thus 3 drives may fail w/o data loss) on the remaining 7 drives.
You then may use zfs snapshots + replication to the other machine or just use a cron job to wol the backup machine and simply run rsync at a certain interval.
[0] https://datatracker.ietf.org/doc/html/rfc6347