This feels like it's probably a no-brainer, but I'm trying to find out the quickest in-game methods

Any diy recipe that uses iron nuggets is a good way to make bells. Each iron nugget used in a recipe is worth 750 selling at Nooks Cranny and doubles in hot items of the day.

The best way to get iron nuggets is to use some NMTs and go on mystery islands tours. Once on a mystery island, hit the rocks there 8 times each. You should get a lot of iron nuggets. You'll get a lot of clay too. If you have a recipe that uses clay, you can make more bells.

I don't know. I find diving kind of boring. My favourite way is fishing. The east coast of my island usually has a lot of fish. I catch as many as I can carry and sell them. Even better when C.J. is visiting.

C.J. is best in the summer when all the sharks are out. I make TONS of bells when I sell about 4-5 sharks to him.

Thanks again you guys, I'll make sure to keep all this in mind!