From what I can tell, you’re only requesting 50 users at a time, why not 100 and instantly half the number of requests you would need?
Also, rather than requesting by username, it might help to request by ID. When requesting using login you wont get data for a user who has changed username, as that login would no longer be correct which would mean another lookup by ID to see what their new username is, so why not just do it all by id and then you’ll have their new username anyway.
Finally, you might want to consider request a rate limit increase. I had my request accepted (still waiting on it to actually happen though) as I also regularly check my user collection for username changes.