I've been working on an Oculus App option for quite a while. I went down the Phonegap/Cordova path a couple of times and always hit a dead end, although I have seen it recommended a couple of times here. But it doesn't seem anyone has actually done it. Just Google Phonegap|cordova Oculus and you will see essentially nothing useful. I fear it isn't possible, especially since we need WebVR capability.
I started building my own apps with Unity and Android studio. For reasons you mentioned I essentially wanted to create a very simple VR web browser app that would run a tour stored locally -- which isn't so simple to do. The best option seemed to be using Geckoview, the basis of Firefox Reality. Essentially I wanted to create a really stripped down version of FxR to run a tour locally. I ran into a problem integrating WebVR. That is still experimental and not documented. And frankly, I was getting in over my head trying to do this kind of project. On top of that, Geckoview seems a little slower than what the Oculus browser offers.
So, my current solution is to run a local web server on the headset or phone, store the tour locally and run it in a browser. FxR is near ideal in that it is so clean visually compared to the Oculus browser and you can set the tour as the homepage, making it almost like a tour app or PWA. But.... you still have that slight performance hit and it isn't available for Gear VR and certainly not iOS. Sideloading it hasn't worked for me on Gear VR -- and I have plenty of experience sideloading other apps -- so I am really perplexed by that. You can sideload it to the Quest and it is available for Vive.
When I started this project I thought it would be simple enough. For me, it hasn't been.