Regarding the problem of repetitive Slack onboarding messages, I recently created an email filter rule that I think should solve it for me. In Gmail filter syntax:

Matches: from:([email protected]) "you have a new direct message" "from your conversation with Slackbot"

Do this: Skip Inbox

This rule might be harmful if Slack ever sends both Slackbot DMs and real-user DMs in the same email, but my guess based on emails I’ve received in the past is that Slack doesn’t do that. Before activating this rule, you can search your own email history to see if any useful emails you received in the past would have been hidden by this rule.