Add scalable geofencing to scavenger hunt app

Hello, I am developing an iOS app and am currently using Mapbox's SDK to display a large map. I am using Firebase's database. I want to: 1.) store GPS coordinates in this database that will show up on the map. Each of these geocoordinates will be associated with an Object (more details on the object later) that will also be stored in the database. This object will contain a thumbnail image. 2.) The thumbnail image should show up at its geocoordinates on the map. 3.) Create Geofences around these geocoordinates (like a 50 foot radius.) 4.) If the user steps inside this geofence (or 50 foot radius) the map icon (the dot that indicates the users' location) should turn green. It should also turn into a button. This button will need to pull information from the object stored on Firebase. I was looking at options to make this scalable, quick, and able to support many users. I was thinking of using Geospark, and will ask you what you suggest.