If you want to make money without cheating by yourself, do island runs. Catch bugs for 5 minutes, return to town, sell them, and go back. You'll learn which bugs sell for the most.
If you want to make money without cheating with friends, go on the turnip grind. Go all in everytime Joan's around, and go on Reddit or some Animal Crossing board and ask to sell at the person who has the highest selling price. Millionaire in 2 or 3 weeks.
I still play this game but since I've owned it since launch I only play for fun now. My town is modded and I spend most of my time giving away items to people or researching new ram hacks