I’m looking for a way to make a Group entry in a sitemap only show lights which are not off, so that I am able to get a quick and easy overview of what’s on at the moment. While I could make a frame with all items in it and then use visibility tags that’s overly complicated and I was hoping that there is an easier way for that. Does such a method exist or do I have to go that complicated way in this case?
Sitemap Group widget is (deliberately) quite dumb thing; you can’t control the order or presentation of member Items at all.
You can build a sitemap frame with individual visibility controls as you describe.
An alternative, which might be more initial work but easier to maintain in future, could be to writ a rule that dynamically adds or removes Items to a “display only” Group Item, scanning members of a “source” Group and usinf their ON/OFF state to determine inclusion.