I don’t know much on the IL2CPP stuff, but I’m going to summon @nirinchev since he’ll probably know.
This is more personal opinion, but I think even if you’re planning to do readonly in the game, you are still introducing risk because you’d have to add an open network access rule to Atlas or wherever you’re choosing to host MongoDB. Just like with giving users only the access that they need, the same would apply to outside client connections.
If you’ve already got a server handling the rest, is there anything preventing you from having that same sever distributing the configuration data via an HTTP request and say a JWT token or similar?
I don’t know anything about the IL2CPP stuff, but I made a game with Realm and Unity for Android and it worked out alright.
Maybe you have another dependency with problems?
Happy to pull in the right people to help you get what you need :-).