I don't think there's any way to know what qualities are currently being prepared. Your best bet might be to just figure out what time that creator typically uploads, and make sure your script doesn't run during that time and for several hours afterward, to give YouTube the time it needs to prepare the high-quality ones.

Could you look at the upload date and wait a certain amount until the processing is more likely to be done?

That's an excellent idea. You can work out a reasonable amount of time to allow YouTube to do the video processing, and then use youtube-dl's "--datebefore" option to only download videos that are at least that old. You'll have to manually compute the date and time for X hours in the past, I suppose.

Thanks for the hint wrt --datebefore. I'll give it a go :)

Ah! That could work indeed. Thanks :)

After it has finished processing, youtube-dl will (by default) download the best format anyway.