Very nice! Unfortunately, maybe because I'm in the U.S., AllRoadMap's slippy map is too slow for me to get a good idea of it, but I understand it from your description. That's basically one of the things I'm aiming for. Thanks!
As for the tags... The MVUM that I view only has a few trail designations. Roads open to highway legal vehicles only, roads open to all vehicles, trails open to vehicles 50" or less in width, and trails open to motorcycles only. This is pretty easy to capture with some of the basic tags for permissive modes of travel. Tracks... Surfaces... maxwidth... date_on / date_off... etc.
I have added/proposed the key:dirtbike:scale
tag. That's probably the only exotic tag I'm using.
I will likely get into relations so that tracks within the Forest Service boundaries are counted properly as Forestry Roads.
My primary goal is to redo the really crappy MVUM though. :-) It just happens that I'm pursuing in a way that all of the work can benefit other projects too.