The user is removed from the Patreon group and therefore no longer has access to the forum (if the forum is only for Patrons).
I really do not think so.
The best thing for me is to have a public category with permissions to @everyone just to reply and see (but not create) topics. In that category, you will write only one topic (you decide how to set it) and your non-patrons users who will access the site will only see that topic (and will have the opportunity to reply in case of problems or to ask questions).
Your patrons belonging to the patreon group will have access to the real contents of the site.