Then I'm all for the solution proposed by Agamnon.
Remove all inclusions from the current main 5e board, so that only threads from that particular board shows up.
Then create a new "all of 5e" board, which shows not only its own threads, but the 5e tagged threads in home-brewed, charop, and so on too.
I've no way to do that, either. I can 'virtually' include a forum in another, but not filter it by tag; or I can do a virtual search for a tag in a single forum or on the boards overall; but I can't do both.