I believe that if the merit is calculated automatically, then it would not make sense. It would be no different from activity. I'm not a big fan of merit, to be honest. Although I agree with the fact that we need to fight against spammers. I would just increase the time for charging activity. This option would be more objective and would not depend on other forum participants. In my opinion, this option is a combination of merit and activity.
Increasing activity requirements to rank up (this is basically what I get from your post), from my perspective won't solve spamming and account farming at all. They would only need to stay longer, keep posting even they don't even read the whole OP, and etc. A lot of people won't care about posting a good post at all because they will rank up either way. I don't see it as a combination of merit and activity too. While I do agree that at some point merit can be a bit subjective, but we can always tell that there is a merit abuse or not.