See this, tbh I just don't get - and I've heard the complaint from plenty of DMs. But if it is soo much time and effort to maintain and schedule a group, why not just play with different players - who actually put some effort into playing?
Admittedly, my current group is online (but previous rl groups have been the same), but Monday night (morning for our international players) is game night. We all keep it free, and if someone can't make it, they let us know. And oif the session is cancelled this week, we'll be back on next week.
But to answer the question, the biggest "work" for me is setting up the adventure in the VTT. Actually running the session is great, it's the prep and the recap that I need to make the effort to carve out time for.