I am afraid you can only limit to a certain board. Not a specific thread. Could be interesting, especially for some “prominent”, thread.
Absolutely not aware on the impact of the underlying database dimension, code required, and overall Dashboard performances, that already look a little bit below expectation from my side (probably we are hitting limits with those above metrics for a public tableau?)

Rn there is no such a tool exist where you can watch total merit transaction of a specific thread/topic from X Numbers of people. But you can get the total merit transaction records topic wise from here - https://loyce.club/Merit/merit.html