I do something similar with my home mail server (not sure I'd go with self-hosting if I started fresh today, but it has run without issue other than maintenance for upgrades & such for well over a decade). I have a second copy running in a low spec VM. Every day it wipes itself down and restores the latest backup, I check regularly (a couple of times per week) to see that it is running and has copies of recent mail (that part could be more automated but I've never go round to it...). It wasn't setup for DR specifically, but it is on a separate network so if push came to shove I could ramp up its resources, change a few DNS settings, open up relevant bits of its firewalling, and have it take over, if the main server failed in a way it hadn't copied yet.