Thank you for reaching out, and welcome to the Community!
You can use the filter "Activities properties > Note > Note Body is known" as an enrollment trigger, and then add an action Send Slack Notification. That way, as soon as a contact will have a note filed in their timeline activity, they will enroll in the workflow.
At this stage activities cannot be set as a re-enrollment trigger. However I can see how it could be useful, so I invite you to upvote this idea on our Ideas forum. It is the main platform for communicating feature requests to our Product Team. You have more information on how this forum works here.