Proposal for tidying up Settings App

could some common settings, such as quiet mode and brightness be separated into a separate 'quick settings' app?

Yes, absolutely. Nothing stops someone making an app that tweaks just those settings - much like someone made a 'Power Off' app recently that allows them to just tap to power off.

Coming up with an easy way to show a grid of icons is on my To-Do list, and it could make apps like this much easier - not that it's that hard using the layout library at the moment: https://www.espruino.com/Bangle.js+Layout