Synthetic user IDs can still qualify as PII since a human may still know which person that ID was associated with and data associated with that ID may be suffic

Synthetic user IDs can still qualify as PII since a human may still know which person that ID was associated with and data associated with that ID may be sufficient to narrow down who it was even if no human (or system) knows the exact mapping. Pseudonymous data is not anonymous data. But yes, the GDPR obviously allows for legal requirements of record keeping. It does however require you to delete the data once those requirements no longer apply (i.e. you have to ensure data is still deleted after those several years, you can't just not delete it because the deletion time is years in the future).