Thanks for the tips. Sounds like I'll just have to redo the Wilbur erosion and try and work out where the issues are coming from. I'm guessing I've used more precipitation erosion than necessary (stupid estuaries) and added too much noise at some point (bumps).

I'm interested in using an exponent too because it definitely looks better than too much fill basins. But I don't know if I'm doing it right. This is how I do it:
Select > From Height > 0 to highest
Filter > Mathematical > Exponent > 1.2

That looks about right, but I'm wondering if starting at 0 without feathering would cause an issue.