The Custom Wizard Plugin looks promising. I’ll have time to try it out next month!
It looks like a wizard could be triggered straight after the user signs up, allowing notification levels to be set per category.
The plugin also allows a user to be added to a group. I’m not entirely sure how Discourse groups work, or how the plugin works with a private group (not publicly visible and/or users can’t join freely) - maybe it sends a group request. Alterernatively, the plugin allows a private message to be sent, so I could use this as a group request.
I’ll look at Discourse Automation too.
Thanks to both of you.