Er, cripes. That was a bit unexpected. I played just one more level, and it turned out to be the last one. It was a little more difficult than previous levels, mainly due to stupid mines you had to slide past or jump over, but still not too difficult. The end boss was another helicopter, who was, again, easy.
So after the credits, I was given a “secret code” (1984), which when typed into the Secret Code option on the title screen, unlocks “Carnage Mode”. Haven’t played that yet though.