IMO, Not all posts in the games and rounds should not be counted in the activity. I mean, I've seen posts that is substantial in there but barely indeed. So if the games and rounds will be excluded then I guess it's fine.
Also for the other posts o other boards, why not just make a rule that these insubstantial posts should be considered as a spam for activity farming and just report it to a mod and immediately be deleted. In that way, the whole board will not be affected but only the shitposts on some threads. Just my opinion.