“were there any co-op shmups for the SNES?”
Aero Fighters (already covered that one) and Firepower 2000. Firepower is one of my favorite SNES games, and it should still be very affordable. Also, it has great content (every level is diverse and introduces new weapons and whatnot) and there’s a dual-layer mechanic of ground/air combat. You can use either the Jeep (stays on the ground) or Helicopter (stays in the air). When on the ground, planes can’t crash into you and vice versa. It’s a very fun game.
Yeah, Firepower 2000 is in my list of games to get.