Would love to know if there was a way to hold off until a threshold or something, but having admin’d servers for quite a while I’ve never heard of such a thing.
When I run unattended-upgrades I normally figure out when I want to reboot, and set it in
// If automatic reboot is enabled and needed, reboot at the specific
// time instead of immediately
// Default: "now"
To figure out the time that is best, just look at yer stats. I don’t see details like that in the default Discourse dashboard, but you should be able to use an external analytics system tell you when there are the least amount of people visiting your site.