I tend to do what the OP described for major updates and add an Update with a timestamp to the original post.
In addition I add in a post to bump the thread and summarise the update itself.
This way I get the benefits of having a single continuous devlog post at the top while also having a thread people can go through. Also get a bump for each update.
I personally don't think this makes the thread too verbose, and I'm not sure bandwidth is *too* much of an issue.
What are your thoughts?