Thinking out loud, but one concept I would find interesting is to link this to groups.
On each chat channel you would have an auto_joined_by_groups option and essentially when a user is created or joining a group we look for associated channels.
I like that it allows to create a custom start experience based on group and also that it will automatically hint to interesting channels when you join a new group.
AFAIK, other platforms tend to just make you join everything you can interact with.