Good remark!
About the join request process
In my opinion, everyone should be able to join a community and there is no need for the approvement of a request. This can be done automatically and will make the join request process a lot easier.
Slack integration
I see the community page as the platform which integrates all the different platforms and communication channels that are being used. It's a valid point you're making that we can think of a better way to integrate communication platforms like Slack. I doubt if changing the button link would fix this because people shouldn't sign up for either the slack, or the community page. An option for this might be to enable moderators to alter the content of the confirmation mail, a link to the Slack channel can be added there?