Khmer are indeed OP, the only change IMO they need is that the farm rate should be adjusted such that it collects as fast as the average of 8 perfectly placed farms around 1 TC, but not any faster. Perhaps get rid of the instant drop-off, such that farmers drop off every 10 food.
I think they might have been buffed too soon, before DE they were only bad on arabia 1v1, but haven’t been explored well enough. They were (and still are) a great pocket civ especially lategame.