What's the most efficient way to make bells?

So paying off the Nook loan and it's a bit of a grind to farm all my island's fruit and resell on another island (where my fruit is non-native) for 500 bells apiece, though that's the fastest/best money I've seen so far. Are there more efficient methods to make bells? Are there different best methods in the early and endgame?

There is currently an exploit using the Mystery Island Tour Tickets that allows for an unusually high spawn rate of Tarantulas, who sell for a high price.

1. Wait for night time (7pm onwards)
2. Purchase a ticket and depart. Make sure to take a shovel and axe, nets can be purchased for Miles once you're there
3. Once you're at the island, pull out every weed, cut every tree, remove every stump, flower and rock in it
4. Drop the aforementioned objects in the sand, along with all your tools but Bug Nets
5. Wait a few minutes
6. Catch as many newly spawned Tarantulas as you can hold, resupplying your Nets as needed

Tarantulas are considered the "fallback bug", as they'll generally spawn when nothing else can. By removing the triggers (flowers, stumps, etc) for almost every single insect in the game, the game will forcibly spawn several Tarantulas every time it needs to repopulate the island.

The only other bugs that will generally spawn are wharf roaches and mole crickets, as their triggers are the rocks by the beach and underground, respectively. Both sell for much less, and roaches can be avoided by dropping items on the rocks.

Mystery Island Tours seem to be quite effective as well, although their lucrativeness is random in quality:

• There does not seem to be a spawn limit on fish in ponds and rivers, so you can efficiently farm for rare and valuable fish, such as the Dorado. That said, there are time and season considerations for this.^Nookipedia

• Similarly, bugs and ocean fish seem to spawn, albeit with a limit, so you can catch and release less valuable bugs or fish and try to spawn more valuable ones. The smaller area, and potentially different spawning rules make this more efficient than doing so on your own island.

• Due to the infinitely regenerating nature of rocks, trees and flowers on the Mystery Islands, you can freely harvest the resources and craft them for profit, especially the daily Hot Items.

• The infinite supply of rocks also gives you access to an infinite supply of ore; red feng shui should improve this.

• There seem to be "bonus islands": I encountered an island with first-generation hybrid flowers and guaranteed Emperor Butterfly spawns.