It’s even worse than this. Backgrounds are also per output device. So if you use one or more external monitors, you need to change the background for each desktop on every single display you use. It’s nuts.
Oh! Thanks for mentioning this. I just realized from testing multiple monitors that the AppleScript iterator tell every desktop actually iterates across monitors. I thought it was supposed to be used for the desktops listed in Mission Control. I just updated my tool to support multiple monitors now, using that iterator. I could only test for 2 monitors since I’m on M1.
tell every desktop