Blomstra will check all your needs
But frankly speaking we have everything what we need with flarum SEO extension. In my case my site (not possible to share links) is ranking first or second for almost every keyword.
Ofcourse for smaller forums we need something like related post it will increase content in the page for all small discussions
If meta keywords are the issue we can add them with help of a new extension
Edit : slug management is needed, seo title magment needed
What else we need?