I suggest bump of topic only happen when somebody who had earned 5 merits in last six month reply on it. It will take power of bumping from those people who got mass chunk in one go and are suspected merit abuser.

If feasible, I will like anti bump button for the users so that they can reply on topic without bumping it.
Anti bump button will be useful when you like to post warnings in thread (so user who are tracking it will read it) but it will not be bumped because bumping might attract lot of shitposter and warning you put in thread will be lost between in that shitposts.


Edit: Another Approach.
Just like source merit, we can have bump merits. Whoever has rank greater than member will get 5 bump merits in a day (if user do not use them then it will expire, so next day its total will be 5 , not more than 5). User will use their bump merit to bump the thread (by default , 1 bump merit is used in each reply unless user decline to use bump merit or already ran out of his bump merits.