I look at this as a fundamentally hard problem of information design. You have a series of posts to present to a reader. Which posts do you give primacy to?

If your reader already knows most of the context and has read the previous posts, then "re-posts" are bad, long-running threads are good, and they just want to see the latest updates.

If your reader is coming to the material cold, a "re-post" may be completely new to them, and posts that presume pages and pages of existing context are completely impenetrable. You want to lean towards fresh, short threads.

The challenge for designing a forum then is balancing the competing needs of those users. You can make some progress by tracking on a per-user basis which comments they've already seen. Reddit does that (maybe just for gold accounts?) where new comments are blue. That makes it pretty easy to skim through a comment thread and see just the new leaves.

But there's still the question of how to sort the threads themselves. It might be interesting if that sorting was also user-specific. Maybe deprioritize threads that you've seen but not interacted with, and prioritize "live" threads that you've participated in and are still changing.