So one of the overarching principles of the highway guidelines (https://wiki.openstreetmap.org/wiki/United_States/2021_Highway_Classification_Guidance#Classification) is that, in general, connectivity should be maintained between roads with the same level, and the level is determined by the connectivity between population centers, not necessarily the amount of traffic.

Since M-72 and M-22 both connect population centers, I think it's equally valid to make Grandview a secondary, or to make M-72/M-22 primary roads.