Why don't you just clear everything, then only select 'unread posts'. This will then only show you additions that you haven't already seen.
OK, so it will still show the ones you're not interested in, but if you look a few times a day you'll only have a dozen or so threads.
You read the ones you want, and they disappear from the list.
When you're left with the ones that don't interest you, simply click on 'Mark all read' and it clears the sheet until next time you look and there's new stuff added.