Fishing for Mythical stuff, what am I doing wrong?

Partly about trawghll, partly about fishing mythics. I have fished in helltide events inside and outside the safety of villages and cities. The only mythic I'm catching in helltides, are fNgs of tathamet. I have caught over 300 of these fish that are supposed to be mythic. But the seem hella common in a helltide. I ain't seen trawghll or a sack of gibbering nonsense either, and he's supposed to be mythic too. I am fishing in torment 3. I have fished the fish out of Shi's haven, and can't pull a mythic there to save my life.

Am I doing something wrong? Is there something else I should be doing before or during fishing? I cast, catch, immediately recast, and then collect the fish I caught while my toon is casting their line. This is irritating me, because I do not understand the mechanics. I'm basically a new diablo 4 player to the current season. My character is a Paladin, level 70, Paragon 43.