I have "Displays have separate spaces" checked, and I can drag a desktop from one display to the other by grabbing the desktop from that "film strip" and dragging it over to the other display. I don't have to drag all the windows individually. They move together with desktop.
The limitation, which you might be bumping into, is you can't drag the current desktop that's visible on a display, which is sometimes annoying but makes sense. Switch to a different desktop first.
I agree with you about the confusing ever-changing number labels on the desktops. I would really like to assign names to desktops, like "Work" and "Project 1". The GUI has room for it, as the full-screen app desktops already have names.
The limitation, which you might be bumping into, is you can't drag the current desktop that's visible on a display, which is sometimes annoying but makes sense. Switch to a different desktop first.
I agree with you about the confusing ever-changing number labels on the desktops. I would really like to assign names to desktops, like "Work" and "Project 1". The GUI has room for it, as the full-screen app desktops already have names.