Agreed, you'll likely want to go with option #2. Dynamic sitemaps are a must when you're dealing with large sites like this. We advise them on all of our clients with larger sites. If your forum content is important for search then these are definitely important to include as the content likely changes often and might be naturally deeper in the architecture.
In general, I'd think of sitemaps from a discoverability perspective instead of a ranking one. The primary goal is to give Googlebot an avenue to crawl your sites content regardless of internal linking structure.