Agreement: Use ISO 3166-1 alpha-3 codes (3-letters ISO code for the country)
ISO 3166-1 alpha-3 codes are three-letter country codes defined in ISO 3166-1.
https://www.iso.org/obp/ui/#search
Reasoning behind
More readability
most used coding system across the FHIR community
3-letter codes are harder for machines to misinterpret
https://andrewwhitby.com/2021/01/08/the-right-country-codes/