Here are a few tips to help you manage your inbox.
Press [⌘A] to select all open messages in the currently selected channel. From there you can bulk action - mark all Done, Snooze all, etc.
[T] or [→] and [←]
Expand / collapse a single item group
[Shift + T]
Expand / collapse all item groups
If you like this setting, let us know, and we can toggle it on for all of your DMs and channels.