i'd say attempt to get an island with roses on it, they typically spawn Man-Faced Stinkbugs, which are worth 1k each, they also have a chance to spawn Emperor Butterflies which go for 4k, they mainly spawn Man-Faced Stinkbugs though. You could also create your own Tarantula Island, but that would require an Axe to chop down the trees. I wouldn't be too reliant on RNG to bless you with a Tarantula Island.

You could also get insanely lucky with a Sturgeon Island, but the chances of that are also very slim.

Man-Faced Stinkbugs as well as Emperor Butterflies spawn from 7p-8a

The overall bells needed to pay off all debts is 5,746,000 unless you add the initial down payment of 49,800 bells which can be payed off in miles, then the overall bells needed would be 5,795,800