For me personally, I often make new range maps in QGIS and import them into iNat. Once the map is imported, you don’t have to guess at the county-level, it’s overlaid on the atlas map and you just turn on the ones that have some portion of the range in them. I will admit it can be some extra work, but with everything already set up and ready, I would say usually less than 20 mins per map.
You can also try contacting the author and asking for their map files, though I’ve had only mixed success with this route.
If you want a bit of a shortcut, you can just georeference the source maps onto county-level maps and skip the iNat upload. Here’s one for Solidaga pallida I did in about 5 minutes (county map from GADM):
As has been said above, I think there’s a balance between atlasing fine and coarse. If it’s very coarse, you potentially lose information and make identifiers upset that you haven’t captured their intent, and if it’s very fine I think you risk misrepresenting the certainty of IDs based on location alone (keeping in mind possible errors both in the range maps and in the iNat observation locations).