Another update, I’ve solved this!
Looks like the error message I got in the last post I made was also a red herring. The to_jid field was in fact correct unlike what the error says, and it’s the user_jid that wasn’t correct – I was passing an ID instead of a JID I believe.
So to recap: the user_jid field that the docs say is optional is in fact optional when you’re making the bot on your own account and authorizing from that account… but seems to be required in all other cases.
Hope that helps others!