I got some help on Discord and am sharing it here in case others are searching for this answer. I was able to catch a Morata fish eventually, and it’s just a very rare occurrence. I didn’t record the exact number of times I went fishing but I feel it was at least 100, and I spent over 30 in-game days just on this. I eventually caught the fish by saving up a bunch of bait, saving the game before fishing, then going fishing. If I didn’t get the fish, I’d load my saved game, spend a day exploring/fighting in order to reset the random number generator, and try again. eventually I caught the fish.
