I'm really casual about bells. I only go to the island if I need extra for a PWP, but I can keep a healthy amount on hand just by selling fossils, miscellaneous furniture/shirts/ore/stuff and hitting the money rock
Honestly, my only method of getting bells now is trading tree bell forum bells with in game bells. It's usually 100 tbt for 10-15 million igb. If I need more bells in game, I just add another 100 tbt to get 20-25 million igb. That's just me being lazy though, but if I'm short on tbt, if you go to the island during night hours you can find a lot of rare bugs. I usually get like 200,000 bells per pocket full every time I come back from the island to sell what I got. Of course, the bells will be shortened if you donate the museum. Also, every day hit every rock to see which one is a money rock. Tip: Before hitting each rock, dig three holes around you to keep yourself from having to step back forward and waste time until it stops dropping rocks
I'm pretty casual when it comes to earning bells. The one time that I managed to earn a lot of bells was when someone posted in the Train Station with Reese buying turnips at 550 bells + per; people are usually very generous and will hold their town at that specific time for a couple of days for people to come sell turnips
I usually hunt beetles on the island for my bells. I'm on AC a lot at night when most of my villagers are already asleep, so I just take myself and Kapp'n to the island to make some money. If I'm playing during the day, I'll usually fish in the river or harvest my fruit trees.
Three ways. The first you mention- perfect fruit, and selling turnips, especially while using another copy to do so. The final is the bell boom ordinance, and selling tons of expensive beetles from the island. If you stick with it, you can make 100's of millions of bells legitly in a couple months.