In the absence of an option currently, would unpinning them not somewhat recover engagement? The “most engaging topics” being “automatically pinned” suggests to me that this is based on something like rate of replies.
If that is the case, I would expect having them unpinned to naturally have a similar effect. Other topics will be pushed ahead of them with every new reply but the topics getting replies most frequently will push those back down again.
It won’t guarantee those most active topics stay at the top but it would at least solve the problem of the latest page being “static and boring” in the short term.