For this, rather than muddying things with custom fields, groups, automation, etc, would sending the new users to the /categories page after signup be an option? Using this theme component Categories Tracking Toggle I think you should be able to add the category notification level option to the categories there, and they can go down the list and pick what they want.
You may need an extra info banner on there too somewhere to provide a prompt.
This reminds me of the Mute all categories by default setup, where people would need to choose some categories to unmute/watch/etc to avoid getting a blank latest page. I don’t think it’s a particularly common setup, but there may be some topics knocking about with tips on how they handle it?