Actually, many higher up/older demons do all of what you describe, and they probably have labs and secret sanctuaries etc. Hordes of minions building stuff for them. They just don’t tell others, or not many others.
I can really only speak for 1’s, 2’s and 3’s since that’s most of my friends. Boggy and I have several joint ventures we run to pass the time. A lot of it involves trading between planes, etc. So I speak for the average Joe Demon on the Street, not enslaved or affiliated with one of the Big Guys. Although likely enslaved to a wizard.
Exador passes the time living in Astlan playing politics, pretending to be a long line of Exadors, sometimes he’s a wizard, sometimes a priest, a noble, a soldier, etc.
Ramses lived there for a while, he’s also spent quite a bit of time recreating Ancient Greece/Egypt hybrid, creating his own demonic satyrs, minotaurs and centaurs etc. (Why will become clearer in Book 2).
But what they really do is “play the game”
All of what you said about infinite unchanging boredom really applies to the gods as well.
So all the higher ups on both sides of the fence play games upon the Planes of Men.
The Quest for the Book is one that Exador, Bess, Ramses are involved in.