I'd say dump as much older content as possible. The "sixty day with no activity" idea is a good one. Keeps the content fresh and prevents the old "we've already talked about that" argument we hear on ALPB forum often.
As for comments...I'd say analyze the content of the comments. When the content is more often, than not, a comment about a fellow commentor, that's where I think the forum gets bogged down. And as much as we'd like to say, "Just ignore that particular person" it does not seem to work. I've been guilty of allowing myself to get drawn into a cat fight with fellow commenters. I've tried very hard to ignore certain people, and it is working, for me, but it seems nearly an impossible task for others.
It is my perception that nearly every important topic on the ALPB is, sooner or later, derailed into a squabble about who said what and why they said it, analysis of motives, chastisement, snark, nastiness, and to use Richard's latest term: "jackassery." It would be nice if we could have relief from the inane musings and speculations of some but those are easier to ignore, I think.
Whatever can be done to mitigate those who participate in this fashion, I say, go for it. Frankly, my participation in the forum is dropping way off because I'm tired of the endless repetitious scoldings and personal attacks, by name, one forum participant in particular, continues to make in what appears to be an obsessive compulsion to attack people personally, ad naseum and ad infinitum. Most recently Richard Johnson called him out and demanded an apology and the only response was more of the same, times two. And again, no action was taken. That this behavior continues unchecked in spite of many moderatorial admonishments seems, to me, to call for decisive and final action.
Beyond that, Peter, I've shared my private observations with you already.