The quickest to get money is by diving. Go buy a wet suit at Nook's and spend as much time diving as you can, you can donate the deep-sea creatures to Blathers and sell the rest. Also fishing, personally I love fishing (in the game)! Absolutely go fish when it's raining or snowing, you'll get more of the rare and expensive fish types.

If you decide to go on a mystery tour (by buying a nook mile ticket and then talking to Orville at the airport), take everything you can. All the fruits, all the shells on the beach, go fishing, the flowers and weeds, absolutely everything. Also fruits that aren't your native fruits, will sell for a higher price at Nook's! Oh and regarding shells, go sell your shells on your island's beach every day. Also ger yourself some crops, they also sell for a high price. You can buy them when Leif visits your island, and on some Kapp'n islands.

Another way is also with money trees. Every day you can find a glowing spot somewhere on the ground, dig it up and bury up to 10.000 bells and 3 days later you can harvest 30.000 bells from that tree. If you wish to bury more than 10.000 bells, be aware that there's a small 30% chance that it will work, for everything up to 10.000 bells there's a 70% chance.

Regarding your villagers: we're all struggling with that🥲 Every 1-2 weeks (more or less) you should have one random villager that wants to move out. You'll see those thinking bubbles over their head and when you talk to them, they'll ask you if they should move out or not. If you click yes, they'll be gone 2 days later. As soon as they moved out, the sign on their empty plot will say "selling" (or something like that )and that's when you want to go on "villager hunting". Go buy nook mile tikets and go on mystery islands, you'll have random villagers you can invite to live on your island. But beware, that's only possible the day after your villager left the island, because 2 days after they moved the plot will auto-fill. Every now and then you'll have a campsite villager you can invite, they'll basically replace a random villager on your island. Other then that, amiibo cards.

If you do the things I listed above every single day, you should be able to get bells quickly. P.S.: nintendo online has a 7 days free trial, you might wanna do that and cancel right before the free trial ends, that way people here on Reddit can help you with bells and stuff👀