This is very important! You can look at your Minerals page in the Collections tab of your inventory to see which ones you're missing, and then compare it to
this table from the Wiki. That way you'll know which Floors will yield the most unfound Minerals, so you're not getting the same ones over and over again.
I'm a Trap Bobber user, and here's the reason: the Legendary Fish are all very "jerky", which means that no matter how hard you try, they aren't going to be in your green bar all the time. And so when they leave that bar and you're trying to catch up, that's when the fish gets the advantage; reducing that advantage is key to success. And I find the the Trap Bobber does a much more reliable job at that then say, the Cork Bobber (which marginally increases your bar size), because there's only so fast that your bar can move.
For the Fishing Buff that you'll want, I personally think that Lobster Bisque is the best, because it gives you +3 Fishing (same as Dish o' the Sea)
and it last all day! Plus, it's really cheap, just 1 Milk and 1 Lobster.
However, the recipe isn't all that accessible. If you don't have 9
s with Willy, you'll need to wait for the 14th of Winter, Year 2 for The Queen of Sauce to share her knowledge.
Second best is Dish o' the Sea IMO (+3 Fishing and lasts 5ms5s, long enough), plus like
@Pangaearocks said, you get 5 as a reward for completing the Specialty Fish Bundle in the Community Center.
But keep at it, that fish will be yours eventually!
If you plan to mine at Skull Cavern any time soon, then I wouldn't bother with getting Rabbits in a hurry. I get
so many Rabbit Feet (Foots?) just from attacking the Serpents that get in my way, although I do spend
a lot of time in Skull Cavern.
But if you've got the cash, it can be fun to have a Rabbit or two around! Plus you'll probably want a Deluxe Coop for it's Auto-Feeder anyways. But yeah, I would definitely prioritise Pigs (you'll need a Truffle, and Pigs are extremely profitable) before worrying about Rabbits.
Hope this helps!