I'm going to build it in React Native and there are some libraries that could help rather than have to use a specific app, eg https://github.com/MacKentoch/react-native-beacons-manager

I don't want to do anything invasive like actually track visitors, I just want to be able to offer helpful "You're near X exhibit" type notifications. Obviously if there's a group close together than it may just become annoying/unnecessary - I'll have to figure that out as I talk to the curators.

QR codes are a good backup and would mean that they wouldn't need an app at all (although I would love to build them one!)