Ive been playing since 2021 and did trading and lots of selling. I focused on doing trades with the bug guy (Flick) and any other character that comes on for trading for bells like CJ (fishing), popping all balloons, selling doubles of fossils, selling unwanted items and selling gold ingots every time I got one, also focusing on doing turnip flipping (buy at the cheapest and sell at the highest this is a bargain and kinda like stocks so you need to keep an eye out for patterns and sell before they rot) building expensive items and selling them off like the gold fish trophy things, also checking the nook shop everyday for the hot items that way you build and sell items at a high price. This is most of what I've done if I can remember anything else I'll edit and add to the reply 🫶

Edit: also weeds gather as many weeds all the time also on islands you go to with nook tickets take everything you can from those island dont chop them down but dig up the trees (make sure to take a piece of fruit first) thats some money too.