Against a mirror-like control deck full of countermagic and removal, the game is decided by who generates card advantage and resolves threats, so Thorn of the Black Rose is excellent: becoming the monarch gives you a persistent card advantage engine, and they have to connect in combat to take it from you. Steel Sabotage stays in the sideboard here. Its only targets are Baleful Strix and Mukotai Ambusher, which is too narrow to justify a slot. Spell Pierce is the weakest link against a deck with plenty of mana that can easily pay for it, so trim it for Thorn of the Black Rose.