One problem that I have is all that all those mods and add-ons are out there, but there's a real mindset that "everything must be an app" and a pursuant mindset that "might as well charge for it". I don't mind paying for a complicated app, but there are certain basic features that used to be served by the freeware model on Macs that just aren't any more, and my impression is that things are heading ever more in that direction. (As well as in the direction of subscriptions over app purchases, which are right out for me for basic utility needs.)

You mentioned Rectangle.app and Hammerspoon, which are both open source. Do you have any good recommendations for where to look for other high-quality open-source mods and add-ons for macOS?

Personally I'm okay with paying for software (though I do whine about it from time to time). Yeah it should just be built in functionality, but someone devoted their time to building a thing, so I don't begrudge them a couple bucks for it - so long as it's a one-time purchase and not a recurring subscription model for unchanging functionality.