That’s pretty easy to do. See step by step below on large terrain that needed to “cut” out the areas of the existing terrain and replace with proposed grading contours/mesh.
Step 1 - You’ve already got this step but I’m showing it as you this is the proposed terrain/grading that I want to insert or cut out of my existing slope.
Step 2 - Draw in 2D the boundary that you want to clip or cut out of you existing terrain. Here I used a combination of proposed grades and retaining walls to create the extent of proposed changes.
Step 3 - Extrude and group the new face created from your extent of changes. Then copy and ‘paste in place’ INSIDE your terrain group. Then select everything while inside the group and right-click and select ‘intersect with selection’. Then hide or freeze your extrusion and delete the areas around your proposed grading mesh.
Step 4 - Do the same thing for your existing terrain. In this example the resulting pink area was the existing to be re-graded so I grouped that and froze it (as you never know when you need to go back and grab something from existing grades).
Step 5 - Move the proposed grades down into place in hole that you clipped out of the existing terrain mesh. Here I’ve left the existing and proposed in together so you can see cut vs fill required.
Finally, here is what the proposed terrain looks like with existing (pink) frozen. Now I can drape and drop trees, buildings, illustrative plan, etc…