Sane people use Debian, Debian packages are compatible with the Debian release they are from. I do not have to worry about accidentally installing an incompatible deb; even if I try to, apt won't allow me to install a package whose deps cannot be satisfied because they're too new (and thus, not in my release's package repo).

I know other distros have problems with release management, but this is why I've used Debian for the past 20 years and will continue to use Debian

Rsync is smart enought to figure out how to get parity.

>What version of the app is running on which instance?

There is always going to be a time during a rollout where there are mixed versions running.