I think the most visible metric is to track the number of topics without a reply and then work backwards to understand why.
You can have very healthy DAU numbers, but if they are just reading and not contributing, then the forum will stagnate. Sometimes a simple reply on a stagnant topic to simplify the question or make it clear that you are seeking collaboration can help.
It might also be that the platform is confusing and does not entice a reply. Forums do require a little more time to interact with given that you want longer more thoughtful replies over an emoji reaction or comment.
We have a support community + community of practice for a brand. I’m working toward measuring if the community helps our members be more successful with our software. Especially against members who don’t participate in the community.