This is a pure glass-cannon mirror where both decks are racing to resolve a reanimated or Show and Tell'd Atraxa/Omniscience payoff, and our sideboard offers nothing that out-races their plan better than our existing mainboard disruption. Thoughtseize, Unmask, and Cabal Therapy are already our best tools to strip their Show and Tell, Aluren, or Force of Will before they combo off, and none of our sideboard cards (Show and Tell, Stronghold Gambit, Koma, Ashen Rider) are fast or disruptive enough to improve on that plan. We keep the full 75 and focus on winning the information/discard race, using Unmask and Thoughtseize proactively rather than reactively.