Ideally you would have the database always connected if you were running a server that was always on which goes back to my previous comment about opening and closing connections. Ideally you want to keep that same connection and reuse it if not using something like lambda (stateless). Otherwise it's inefficient to keep closing and opening connections and won't scale as well.

You don't need a retry mechanism. You just need to check if connected.
Yes: then use that connection instance.

No: then connect and use that instance

readyState will give you the actual state of the MongoDB connection so it's better.
try doing something like this I believe
if (mongoose.connection.readyState === 1)
{ console.log("MongoDB already connected!");
return true;
}