For anyone interested I figured out a solution! It was very simple once I realized I needed to mess with Y values. Vertical stairs don't require anything to look nice, just the slowdown.
I applied slowdown, same as vertical stairs, to my horizontal stairs using their own trigger box. When I'm overlapping the trigger my character's Y value is increased or decreased very slightly depending on which way the stairs are facing and if you're going up or down them. Here is the code I came up with. It says +/- 0.1 in the code, but making it +/- 0.2 makes it much, much smoother.