That’s the easiest and fastest way to do it using fields. On the other hand, you could create a custom taxonomy that is hierarchical and then at the top level create the states and then on the second level you could have counties.
I would also carefully name my county fields. If you are using the state abbr for the value of state then:
ia_county, nm_county, etc
then you can simplify getting the county
$county_field = $state.'_county'; $county = get_field($county_field)
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