One thing to consider is how many of your users regularly log in and visit, and whether you need to reach users who don’t regularly do that (who may normally just stay in the loop via the activity summary or read emails when they are mentioned, etc).
If the people you need to reach visit regularly, the banner, custom home page design, or pinned topic approach likely works well.
If there are a sizeable number of people you need to reach who don’t log in regularly, something like the category with “watching first post” set may work better.
Whatever you end up doing, would love to hear what you try and how you feel it’s working later!