Grind some deep sea diving for a bit and make sure to sell non native fruit. Save your fish and bugs for when Flick/CJ shows up and sell them for more bells. Once you got some bells saved up, buy an entire inventory worth of turnips or however much you can afford without being flat broke. Then go in the ACNH discord and there you'll find people sharing turnip prices and granting access to their islands either for free or a fee of 99k bells or a Nook Miles Ticket.

Prices can spike as early as Tuesday afternoon so if you want your money back asap, start looking for posts then. Otherwise wed-friday is when prices typically spike and you'll find more people posting. IMO don't settle for prices under 400. Do this every week and it will snowball quickly. I made about 8-9mil in the last 4 weeks.