Yeah, it sounds simple, but when you start picking at the idea...
First of all, won't the game have to essentially delete itself to replace its own files with the updates? Will that affect the user experience (eg. the app suddenly just auto-closes and must be rebooted ??).
I figured for a nw.js thing you might be able to just download some pieces of it (eg. just the new JS files and some new PNG assets, etc). But that might be tricky on Mac, where all of the project bits are "inside" the application itself. So I don't know if that would work.
Maybe it would be better to make a launcher program that checks for updates and then runs the game?
Or maybe the simplest thing is (despite saying I didn't want to be on Steam or whatever) to find a distribution platform after all that would handle updates for me. I hate using Steam's publishing interface but I guess from the user's point of view that might be the most seamless way to receive app updates. 🤔
Anyway, the game in question won't be done for a while yet so I have some time to mull it over. Still very much open to ideas and suggestions!