_{Contributions originally left as comments}

I typically aggregate dynamic columns into a single JSON value these days rather than creating dynamic columns for each attribute. That might be a challenge though as well, with your outdated (and unsupported) Postgres version - a-horse-with-no-name

The desired output can't be obtained directly in SQL. You need to write a query that generates the query that generates your desired output. Giving up on your desired output and producing JSON instead may also be considered, as it would be simpler. - Daniel Vérité