I make over 200k bells per evening catching beetles on the island.
Basically I've cut down all trees except for two palm trees on either side of the island, and I wait until 7pm when all grasshoppers have gone to bed (which usually fits in with my work schedule anyway), first I pick up all shells off the ground so it's easier to see if a hermit crab has materialised, and I run between the two Palm trees scaring off all the wharf reaches/tiger beetles (I think? The other bug on the ground that isn't roaches) and hermit crabs as I go, and catching the stag beetles from the palm trees. Once I've filled my boxes with beetles I go home and sell them for 200k +