Having said all that, I don't know the answer to your question as to whether any radar computer tries to correct for slant angle
The Calc5,
Item 22 in the picture, of the Navigation Bombing System (NBS) fitted to the V-bombers did just that. It was analogue and consisted of a pair of servomotors that moved along tracks set at 90 degrees with a thin metal strip runnning between them. The strip was kept taught by a reel mechanism. The vertical servo moved up and down according to the aircraft height and the horizontal servo moved until the resistance in the metal strip was propotional to the range to the response on the screen. The position of the horizontal servo was then sent as a correction to the display range on the screen. If it could be done all those years ago with the next thing up from clockwork I am sure that a box of "1"s and "0"s can do it better today.
I suspect that some correction is applied to processed ATC radars. Best ask in the ATC section.
YS