I am successfully triggering on "Activity is added; Trigger for Activity Source" and updating the status of the activity. Now I need to assign the activity to the Source contact in the activity so they get the email notification with the activity details. Can't find an action to do that. I can send an email, but that lacks all the activity's details.
I do not think there is an action yet to do what you want it to do. It is not a complicated action though, so it should be relatively easy to add. Do you have any PHP skills? If so, it would be great if you could do a PR and add the action? There is documentation on how to create an action here: https://docs.civicrm.org/civirules/en/latest/create-your-own-action/