If the same slope angle is to be created again, then I would virtually remove a cuboid 2.5m x 35m x 3.05m at the back (which must be disposed of) and push the slope 2.5m backwards. I could lend a few building blocks of my child for better visualization.
So I don't think the 755.9m³ is correct.
Yes, it is correct. Initially, you have only created a hole where the slope has space - but it is still in its old place for you, so it must be removed
and rebuilt on your remaining embankment.
One must be careful here about what is actually being
moved - the total volume is, if you remove it in front and rebuild it at the back, ultimately even the same; which of course does not mean that the moved volume is zero.
Best regards Dirk Grafe