Let me suggest a different starting point. Use your mask as a mask. Don’t trace the outline with a layout generator. Just look at masking a noise as your first step.
You should see something that fairly accurately follows your mask perimeter, but with that nasty height jump at the “coastline” you mentioned. There are many ways to deal with that. One is to go back to your drawing program, and modify your mask with a blur or feather or whatever your program provides such that the “coastline” is black and there is a grey scale ramping up to white over a modest distance “inland”. Then apply the modified mask to a noise and you should see the “coastline” at altitude zero, ramping up to the full noisy amplitude “inland”.
That should work as a starting point for you, with much less labor and more reproducible results.
Once you get more comfortable with WM nodes, you should be able to do the coastline-to-inland ramp modification in WM itself, without having to modify the black/white mask in your external drawing program.