There is a way to look at the slacking part more simply, I think - if you have an employee that you sincerely have no way to tell if they are producing anything

There is a way to look at the slacking part more simply, I think - if you have an employee that you sincerely have no way to tell if they are producing anything or not, and neither can anyone else at your company, then why do you have them? What was the job description meant to have been when you hired them and how did you end up here? A solution for this could be managing to outcomes, and setting more clear expectations around their role.