That particular example of a river crossing over a highland was because I used too much noise, more than half of a terrace step. More noise = more wiggles on rivers. Less noise = straight lines.

Wilbur's deterrace tool uses a raycasting algorithm that casts rays in a circle to find the distance and angle from the current point to the next point that's not on the same level. Those distances and angles are used to calculate the theoretical altitude at the point. There are two parameters, the altitude difference test value (in the registry under "Deterrace", "Epsilon", default=0.00001) and the number of rays around the circle (in the registry under "Deterrace", "Rays", default=64). It has some unpleasant behaviors such as always wrapping in X but not Y (it was intended for whole-world operations). For your particular image, you would need to increase the epsilon value substantially because the base parameter assumes effectively flat areas and your "flat" areas are decidedly not flat.