How to open this semi flush glass disk light fixture?

I'm not sure how to add images to these posts, so I uploaded a couple here:

Usually those require that one gives the "globe" a firm pull down. The base should have some retention tabs that hold the globe in place.

When I pull down, the whole thing (including the base) starts to pull away from the ceiling. It feels like I'm about to rip the whole thing out of the dry wall...

It could be that it is one piece. I have not messed around with that particular model. You stated that the base rotated. Can you see the lamp holder rotate with the base?

Your photo looks like the lamp in our kitchen, except that ours has no flat disk between the fixture and the curved light diffuser. Our diffuser removes by rotating, i.e. locks to the fixture by lugs like those that hold together the two halves of a smoke detector. When we installed it both my wife and I practised removing the diffuser, so either can replace bulbs when we need to.

If your fixture moves when you try to turn the diffuser, the fixture was probably improperly fastened to the cieling. It may be safer to pay for an electrician's house call.

Since this is not a binaries group you provided the images in the perfect way.

Now, as far as the disassembly of the fixture is concerned, if rotating the glass portion appears to rotate the complete fixture, the only logical solution seems to pull straight down harder than you already have. If the electrician installed everything correctly I would doubt you would pull the box through the drywall. But that could be a big "if."

Good luck.

All the old "pan" fixtures in my daughter's townhouse required a counterclockwize turn of several degrees, followed by a pull down. Some were nastier than others (and all were full of dead flies when she bought the house)