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

As noted in that thread, there is no property indicating an item is in the reading list. You can only query the reading list and make comparisions from that list.
You instead may want to consider using a tag or other marker to indicate a file will be read later. This could be queried more directly, e.g., tags:"to read".