You're right, we had a pretty similar idea. I would even say that your concept is better than mine.
I like the fact that you can do so many things on the phone, not just looking through the camera like in my case.
You made your game in unreal engine, right? I'm interested in how you solved the problem of the ghost that can't be seen with the naked eye but can be seen on camera. The way I achieved this was I gave the ghosts a translucent material that is able to collect custom depth and then I used a stencil so that in the post process of the camera I could make the spirits