recommended approach to generalising code with variables that must be assigned by the user

I think you want to add a custom.html: that can open a dialog when uploading the app, ask for those settings, and send them to the watch.

Looks like hasensors does pretty much the thing you want: asks for a bunch of values, then replaces {placeholders} in the code. (Except you probably don't need boot.js, and want to adjust your app.js instead of using a separate lib.js file)