The issue was with using pg8000.native

* I switched over to importing plain old pg8000
* Changed the SQL value placeholders from ?/$1 to %s
* Switched conn.run() to .execute() after creating a 'cursor' object:

cursor = conn.cursor()

cursor.execute(INSERT_SQL, params)

I never set out to use pg8000.native, but did it upon the suggestion of a chatbot after psycopg2 broke a different part of my pipeline design (I am not ready to start learning about containerisation today with this burnt-out brain!).

Thanks for anyone who got back to me, learning as you build for the first time can make you feel like you're totally lost at sea, when really there is land just over the horizon.