You have to switch off Terraforming (-1) no runway profile, no flatten airport, so the underlying terrain profile will be used. To make own slopes in any direction the “easyist” way is to use an heightmap. Best with smaller radius maybe 5m and 5m falloff, then inflate or make sinks with STRG with very weak values . After that smoothing. For better watching switch on isoline view and make the heighmap transparent. This is very tedious, but unfortunately there is no direct DEM Import so far.
Good luck