According to some data I collected, you can make ~2300 bells/min diving, which means 500k would take a little less than 4 hours.

https://docs.google.com/spreadsheets/d/12psfVrErw9okyAP8Dr9DX7-B7wuh6g5E1Vq3g-H65_I/edit?usp=drivesdk

Darmok and Jalad at Tanagra

I think your server will only download content if one of your users views/subscribes to it, so maybe you could disable signups so that you’re the only user on your server? This would still allow users on other servers to subscribe to your communities.