Tackling the terrain of my worldbuilding project

[waldronate]

Precipiton erosion in Wilbur moves altitude from high to low. If the oceans are well below 0, then the land (at or above 0) will get smaller because some of the altitude is moved below 0. If the oceans are at (or a very, very small below) 0, then land will get larger as altitude builds up around the edges. If you keep a selection and run the oceans at 0, the land will hit the edges of the selection and stop. In this case, areas of rapid altitude transport will grow cliffs because Wilbur is doing the precipiton erosion and then blending that result back to the main surface through the selection. If you keep the ocean at 0 until the last run, then set the ocean down a bit on that last run, you'll get nice and crinkly edges. If you keep the ocean down and use height clip with low=0.01 and high=10000000 after every erosion pass, it will ensure that your coastline always exactly matches your mask.
One trick to force rivers to where you want them to be is to make a mask that has just your rivers on it. Use that mask to punch down the rivers (load the river mask as a selection and either set to 0 for subtract some amount, the load back your coastline selection) once in a while and the precipiton erosion algorithm will keep finding those rivers as the low spots, forcing the rivers to flow that way. They may eventually become ingrained enough that you might not need to keep punching them down a lot.
Erosion in Wilbur is a relatively untamed beast: as long as you keep a close eye on it and regularly apply the lash of masks as needed, it will do a lot of the heavy lifting of detail creation for you. If you let it do its own thing without much supervision, it will try very hard to get to its desired end state: a flat plane.