If you disable the wheel brake on the MI-8 and try to taxi forward it will roll backwards on flat ground. (For wheeled helicopters they should be able to essentially “drive on the ground” or taxi without needing to take off to do it)

I suggest that simply moving the cyclic forward or backwards should control the speed of the taxi like it does irl. Currently in game you can really only taxi going forward if you do a rolling landing (land at forward speed like a plane).