An other thing you can do is attempt to improve your content. It's user generated so it's not that easy, but when someone searches for information, often they don't want a load of off topic guff, people bickering etc.
You can do quite a lot of forum curating to help SEO.
- Encourage on-topic posting (discourage or move off topic posts from within important threads)
- Merge threads that are about basically the same things (similar threads addon is good)
- Encourage people not to start a new thread about the same topic as an existing one.
- Get rid of trolls.
- Encourage meaningful thread titles that include words that are right on topic.
- Have easy navigation so that people who find your forum are encouraged to stay there (engaged) and look around.
- Search for old threads that only have one post. I just delete them because usually it's a question that nobody answered, so it's useless to Google.
- Not just threads, post some useful well written articles on pages or resources., structure them like proper articles with headings and subheadings, articulate writing, good original clear images, internal and external linking.
- Look at the competition and work out why they do better in Google.
- Think about search questions.
It's true I don't think there's any great xenforo SEO addon (equivalent of Yoast) , I'm waiting for that but meanwhile just do your best at having great content.