My point is that mapping this as a separated cycleway while deleting the tags from the road: 1. removes information (notably, that this is buffered, and not grade or hardscape separated) and 2. is strange for routing

Could you point me to where you see those guidelines on mapping as separate cycleways? Based on https://wiki.openstreetmap.org/wiki/Bicycle and the cycleway disambiguation page, I would still only see hardscape or grade separation as suggesting separate mapping