Sorry to be spamming here but check out my https://github.com/mherrmann/fbs. It is literally the solution to your pain point.
In order to use this, if I read the tutorial[1] code correctly, I must modify my app to wrap everything -- or at any rate, all PyQt-related things? -- inside an AppContext, and modify the outermost level to create this.
If so, this is easy enough for the tutorial but what if my app opens multiple windows and has code to create them spread over multiple modules (as one of mine does). Is there a simple way to get all these bits into one "context"? Or have I read too hastily?
Yes you should use the AppContext. You don't have to convert your entire app to it. Only enough to "start" your app. (I highly recommend the app context approach where possible however, it's just so clear.)