Oh boy, I was really triggered me with this thread!
So I decided to have another go at doing an offline capable mobile web app using UIBUILDER.
But this time, instead of trying to do the horrible, complex manifest and worker's, I used Vite with the vite-plugin-pwa plugin which uses Google Workbox to do some of the heavy lifting.
A couple of hours later - shazam! I have a working (very simplistic) version.