According to some data I collected, you can make ~2300 bells/min diving, which means 500k would take a little less than 4 hours.
docs.google.com/spreadsheets/d/…/edit?usp=drivesd…

ACNH bell earning rates
Sheet1
bells,time (mins),bells / min
fishing #1 (cj),18000,10.75,1674.418605
fishing #2 (cj),25800,16.2,1592.592593
diving #1,24500,12.25,2000
52300,19.75,2648.101266
Google DocsThere is research showing that adding lanes only helps for maybe six months. Then people realize that the route is better and change the routes they take, which leads to more congestion again. Fewer lanes can actually decrease congestion.
smv.org/…/how-does-roadway-expansion-cause-more-t…

How Does Roadway Expansion Cause More Traffic? | Blog | Science Museum of Virginia
Research suggests this counterintuitive reality: adding more lanes to roads actually increases traffic congestion. Yes, you read correctly: expanding highways does not provide a long-term solution to traffic congestion.
Science Museum of Virginia | For curious-minds of all ages!What have they ever done for us?

Checked Exceptions are Evil
The concept of checked exceptions is questionable. They lead to breaking signatures, scattered logic and boilerplate. We'll see what we can do instead.
Philipp Hauer's BlogDarmok and Jalad at Tanagra
I think your server will only download content if one of your users 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.