I'll be adding those catch rates into the game inside of the fishpedia soon, since it would definitely be a great thing to see how likely a fish is. Otherwise all the values are pretty random, but legendary fish are usually around a 1% drop rate.
Right now the improved bait upgrade only changes catch rate, though I have some plans on how to make that mechanic a lot more interesting. I've thought about adding in different types of lures, that give you different abilities when fishing. Like one that only catches big fish, or one that increases the chance for legendary fish. That way people have a lot more control over what fish they're trying to catch.
that sounds awesome! i love the game so far! the game is super fun and i love the inclusion of the mini games! it makes the experience much more entertaining than just the typical fishing game. i would love the inclusion of lures, and i can't wait to see what else you have in store for players to come!