Testing usage of PARALLEL SAFE to see if it improves performance (and testing to be sure it doesn't have any negative effects).
I decided it was probably safe to set "PARALLEL SAFE" on functions containing CTEs, despite vague messaging in Postgres manual, based on this discussion: https://www.postgresql.org/message-id/CAGHENJ6dF12ewNS%3DbrvGfafi8tPu6WQ2usati7Occqni-htSnw%40mail.gmail.com
Anyways, so far it's far from clear yet whether allowing parallelizing helps or harms in our current queries in terms of performance.