many of the tags don't seem to adequately cover the permitted use

Here in the US, public lands mangers deal with motorcycles, atv, 4WD etc with a single term

Off Highway Vehicle or OHV

There are OHV parks run by the US State of California, BLM, Forest Service, private property owners, and other government agencies

These vehicles are generally not permitted on general purpose roads though not always, connections are sometimes allowed, they are not subject to certain vehicle registration, insurance, equipment, and operator requirements

here 4WD doesn't cover it as some OHVs are 2WD and perfectly capable of proceeding, some are only passable by motorcycle or highly modified jeeps etc

Seems like an OHV tag would be useful.

another designation under access would be helpful too

Many times while a certain use is permitted it is not recommended

access=not_recommended would better cover certain situations