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I have rarely seen adding or removing sitemaps to dramatically impact a website.
Maybe I'm wrong but from your introduction I understood that you don't want to continue with the sitemaps mechanic because by adding a forum it may become a total mess.
Personally, I think sitemaps.xml files are EXTREMELY overrated
For a forum, you may actually be better off not having a sitemap. It isn't just adding material. You have to keep track of every thread that is moved/ merged/ deleted and adjust the sitemap accordingly.
there is no way I could keep up with a sitemap including a forum