Is there a way to see (property) or search for (smart group) items that are (not) in the reading list?

Yes, since — as far as I know — the Unread status is switched off as soon as I select/preview the item and for me, that does not mean that I read the item. I want the status “to read” to be more “sticky”, it should require an action by me. But thanks for the hint, I did not think of it since I don’t use it at all.