" In all my time of coding, there has only been one situation in which I used cosine: games, games, games.

  function move(obj) {
      obj.x += obj.speed * Math.sin(obj.rotation);
      obj.y += obj.speed * Math.cos(obj.rotation);
  }

"

Why not store the velocity as a 2D vector instead? Then you still have to use cos/sin to compute this vector, but at least you don't need it every frame, plus often you don't need to use cos/sin to compute this vector either since forces that act on the velocity themselves can have an x and y component you can directly add to it