Great approach.

Worth noting that there is a 6 decimal precision on the coordinates of the 90kb (gz) `coord2state.min.js` ... which suggests an accuracy that may not be present in the simplified data (i.e. <1m).

Before you increase tolerance to decrease filesize, you could consider lowering this decimal precision to 5, 4 or even 3 decimals given the "country, state, or city" requirement.

I also like the idea of using a heavily cached, heavily compressed image that is perfect for the >95% of the country that isn't within a pixel of a border. With a subsequent request for another heavily cached vector tile that encompasses any lat/lng within your 1px tolerance.