Just found your post today. . . . .I too purchased a Ferris Evolution this year also, 48" and have a ditch line similar to your photo except not quite as severe. I also noticed the mower had a tendency to want to slide when running parallel to the ditch line and countered by crossing the ditch line in a perpendicular direction. That change worked well to a point where the ditch approaches a "V" shape. Then it's time to break out the string trimmer to finish the job.
I bought the Evolution because of the suspension system and can't believe how comfortable it is.