You don't need to catch any of the 400+ gathering "living legends" required for Feast of Famine to get the i80 rod and 106/106 achievement. There are 112 big fish, so the six needed for Feast of Famine conveniently are not required for Go Big or Go Home X. I'm fairly certain they did this intentionally - you get the 106 other big fish, get the new rod that gives you enough gathering to get the last 6, then complete Feast of Famine.

Pretty sure at this point Feast of Famine works like this:

Catch Namitaro at Urth's Gift, Endoceras at Isle of Umbra South Shore, and Helicoprion at Sagolii Desert for the Black Shroud/La Noscea/Thanalan part of the quest. Then catch Kuno the Killer at North Shards, and Shinosaurus at Witchdrop for the Coerthas/Mor Dhona part. Then catch Nepto Dragon for the final part.