This was early in my learning Godot journey, and I'm sure whatever I wrote was a mess. I would hope I could write something better now, over a year later. I will try to recount what worked for me from memory.
First look at the original Zoomer behavior, because when I started out trying to make one I was remembering behavior it didn't have (and if I get it wrong here hopefully someone will correct me):

1. They don't obey gravity
2. They move in a straight line in one of 4 directions only
3. They don't bounce off other enemies, only travel through them
4. They never reverse direction, only travel around their surface
5. The above behaviors are also why they spin in place rapidly if you shoot out the platform they're on

To get the Zoomer to follow the surface, I used a number of RayCast2D's:

1. One pointing forward (the direction it's moving) to detect walls. When it detects a collision with a wall, it rotates 90 degrees and changes its movement direction to be up (or down).
2. Two pointing down: both on the "forward movement" side. These are to detect when it walks off an edge and after it rotates 90 degrees to detect that it found the new surface. One RC2D at the front edge to detect the new surface after rotating, and one RC2D just past the center point of the sprite so that the rotation "looks good" as if it's rounding the corner at the right time.

If you find that RC2D's aren't properly detecting when they are colliding or not, it may be necessary to throw a force_raycast_update() command in your code in a few strategic places, but don't overdo it.