On your first extra turn, do it again: cast Nexus of Fate for an extra turn, then tutor it back out of your library with Mystical Tutor
My reasoning is twofold: first, that (IMO) targeted discard is most effective against combo, which is nonexistent in my cube, therefore its not strictly needed in the environment to keep unfair decks in check
If they do, tap this creature and create a 1/1 blue Fish creature token with "This token can't be blocked." Creature - Kraken Color tag templates(None) Serpent of Yawning Depths Krakens, Leviathans, Octopuses, and Serpents you control can't be blocked except by Krakens, Leviathans, Octopuses, and Serpents
So this can be a very powerful card draw/recursion tool
More and more Magic cards are printed every year, and in 2021, with fresh Commander decks for each Standard set, the Modern Horizons sequel, the new digital-only Alchemy cards, and so much more, we're now firmly at a point where keeping up with everything is downright daunting