I know coincidence is not causation, so I want to be clear that I don't think the valuable information you provided in your post about decluttering PixInsight caused my toolbars to disappear. But your discussion does suggest you might be able to help me figure out why. I've google my brains out and posted on PixInsight and Cloudy Nights forums, but so far no joy.
After several weeks without the problem, projects no load without toolbars. If I reset to factory default or create a custom arrangement as you suggested, a saved project reloads without toolbars. If reconfigure everything I can happily go about my business. But if I save the project, with or without images, the load without toolbars.
27" Mac with MacOS 12.4 (12.5 as of this morning) and PixInsight 1.8.9-1