So adding apache as a proxy to the Discourse nginx is definitely an option.
I agree that a pro would be making future upgrades easier, and that’s an important point.
But adding another hop to the architecture would not only make debugging issues in the future more complex – I also have concerns about Apache’s performance as a proxy to a web application that uses long polling, as pointed out by @sam in this post from 2016.
I generally prefer nginx to apache, except when it comes to mod_security. It would be fantastic if the OS repos included packages to enable mod_security in nginx like they do for Apache, but currently enabling mod_security on nginx requires compiling nginx from source in both RHEL/Cent and Debian. And I avoid depending on packages compiled from source on production like the plague…