I know that going to the island at night and getting sharks/beetles is a solid way to make money, but I feel like there's some other tips and tricks that I've missed out on. What are some of the ways you guys make money quickly?
Crowns. Sell them if you have that many since they fetch for 300,000 a piece. Could be more if ordinance is bell boom and when it is a featured item.
But it costs more to buy crowns unless you hack them.
or got them free from people
Someone may as well just drop bells in your town then. The OP asked how to make bells, and getting bells / crowns from someone is not really a plan unless someone is feeling really generous.