After spending my fish cookies to upgrade my fishing gear in the Autumn Cabin, I've noticed there's virtually no benefit to it. I've now got the Bamboo Rod and the Max-Level Hook, and I’ve been fishing with them side by side against the default rod, but there’s hardly any difference.
The rod upgrade seems to let me hold slightly more creatures, but the fish I catch don’t sell for any more, and the rarity of what I'm pulling in hasn't changed at all. The biggest issue is with the hook. It's supposed to increase the bite rate from 1 to 5, but I can't tell a difference. The time it takes for a fish to bite is exactly the same as with the basic hook. It feels like a waste of the currency I worked to earn.