When I run into this situation I end up making an additional model landscape component in my model that runs from the back of the curb about 10’ or so and is several feet thick.
Then I assign the same landscape material I will be painting and sculpting on the terrain to the 10’ landscape component i made.
Finally I sculpt the landscape terrain to the landscape component. It is much easier to sculpt and paint to this way not having to have a perfectly sculpted hard edge like you do sculpting to the back of a curb.
PS Sculpting seems tome to be better used in large landscape areas and detailed landscapes still need to be modeled.