To make a nation-wide accuracy map, you'd need to compile property data from every county (or equivalent) in the nation. Assuming every county publishes it publicly (I'm sure some don't), it would still be a ton of work...

The real catch is this, though: if you actually pulled it off, you would have just built a nearly flawless reverse geocoder, and you wouldn't need to use an external API at all. You could just look locations up in your huge property data set.

couldn’t the OP use the same method to determine accuracy that they used to compare providers?

it doesn’t need to be an accurate decider, just “good enough” in a majority of cases.

The methodology used to determine accuracy involved loading GIS data on all property in a given county. Therefore building a nation-wide accuracy map using that methodology would, by necessity, involve loading all property data nation-wide. And if you had all property data nation-wide, you wouldn't need a reverse geocoder. You'd have one.

We've put together an (objective) comparison tool here: https://www.geocod.io/compare/