The beauty of quasar is deploying to multiple platforms form a single codebase.
Regarding PWA caching: all this does is keep in memory the static assets that make up an SPA, so you can access those offline. In other words, a PWA does not help with user initiated server interactions. So if you want your app to work offline and still register user actions somehow, you need to figure it out on your own.
So in the end I solved it with the phonegap-plugin-barcodescanner, which is very similar to the cordova-plugin-barcodescanner, but you can configure it, so it feels more like a QR scanner.
Be aware that it the two above internally (in cordova) register/use the same name, so before changing from the cordova plugin to the phonegap one, remove it with the command below before adding the another one, so the build process won’t break.
$ cordova plugin rm cordova-plugin-barcodescanner