How can I flatten an area in a surface seamlessly?

Hi, I am trying to model the landscape for a house. I’m quite happy with how the terrain is looking, but I’m having trouble with the area around the stairs. I would need to flatten it out but I want to do it in a natural way.
Because I have already extruded the surface, then also carved out the site and added the pathways for the garage, if I explode the terrain and try to work with control points all my previous work will be lost.
Is there a way to carve out this flatten area with boolean operations, but then smooth out the transition to make it seemless?
Thank you very much

Dear @Elisa_Goldfarb
if you want to modify the Control Points (cvs) - you can select a bunch of cvs and use
_setPt (x no y no z yes, world)
or you can orient the gumball to world
and den use the z-scale handle and scale with 0

another option is to cut / trim out a certain area with a curve - most simple from top view
then insert a new (planar) surface and use _blendSrf for the transition.

does this help ?
kind regards -tom