Hi and welcome! I see you have no previous edits on OSM. As a fellow gravel-riding OSM editor I thought I would chip in here with my knowledge.
As per your description, I would highly recommend considering obstacle=vegetation, which is quite a good tag for overgrown tracks where riding may be hard/impossible but walking is still possible, even if with difficulty. Also consider tracktype=grade3 to grade5, depending on your judgement, this tag indicates the hardness v softness quality of tracks (maatalous/metsätie). For wet parts of the track consider surface=mud. If I remember correctly, mtb:scale should only reflect the surface condition of the road, not surrounding vegetation or surface material - other tags that I suggested take care of these aspects - so I would keep that in mind as well. If you want further information I would check the wiki pages for each of these tags, loads more information on there than I could write here.