The toughest part is, the regulation talks about the definition of personal data here: https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX%3A...
‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person;
The keyword people like to use is "in context", where data is personally-identifiable if it can be used to identify you in a certain context. This... covers almost everything with a unique=TRUE in a database, and any ranking-type systems such as the excellent example of MMR you gave earlier.