I don't really understand what you need, a crater or a mountain? It's a mountain with this negative displacement. But you can play with the offset to raise or lower the whole area. Or you could use a surface shader under the initial terrain, masked by the simple shape, set it to smooth terrain (it'll annihilate all previous displacement until it hits a compute terrain), and then add your second PF terrain masked by the same SSS or maybe as a child to that surface shader.