If you want a Kanban board…
Make a private category assigned to a private group. Makes no sense in regards to personal messages… cause then they aren’t private.
Maybe I don’t understand the use case. But, an invite-only group-only assigned category will keep anyone else from accessing the Kanban board without your express permission. You can then assign a private tag grouping invisible to the rest of your forum.
In addition you could assign a custom 3rd party email (not your Discoure default) for group related messaging. Makes for an excellent private sub-forum and you can add in this Kanban theme to it if you have administrator assistance.
You can also make your group, or a related group, moderators of your private category.