I think what you talk about are fallows. This is a integral part of crop circulation systems, a land use that gives the ground (and its eco system) time to regenerate. This is to be distinguished from dedicated meadows that is harvested for grass or hay or used as pasture, and also from natural=grassland, which is uncultivated land.

I do not think that remapping every year makes any sense. You would document a step in a cycle which is short sighted.

Why not just tag it as farmland, and leave out the crop? You would not know from pictures anyway.
IF you know, I’d propose to use crop_rotation=yes, crop=;;fallow

This would document the real land use more precise that micro mapping, and would also reflect both the pattern of land use and fact that seemingly unused land is part of a cultivation form and thus actively in use.