As each battery is in a pre-designated location it should be possible to sub-set and pre-process the DEM to eliminate unnecessary 'real-time' calculations from the whole dataset... only once the mobile SAM/AAA positions are modelled in a tactical situation where they can move is true real-time processing essential.
Each battery would then have a minimum elevation for no interference and then unique patterns of interference for main and side-lobe noise for their elevation/azimuth ranges. OTOH the storage requirements for a full detail pattern might be prohibitive, but it may be worth considering??

It is done that way.
The units are elevated to their realistic height, to be able to look-down, and a pre calculated "panorama" picture is applied.
The target planes flying under 3000m got their terrain following radar to realistically fly above terrain.