There are discords, both in English and Japanese, where extensive trial and error testing is done to figure these things out. Right now, six of the eight new big fish have already been (mostly) figured out, just through sheer manpower and dedication. After things are figured out, the catch data tends to slowly trickle down onto sites like ff14angler and the community at large fills in whatever gaps may have been missing in time frames or alternate weather/bait what have you. Datamining doesn't tell you how the fish are caught, and at best only tells someone whether Fish Eyes works at a spot or not, something any player can and should do themselves when first starting to look for big fish anyways.