Well, just remember that s2Member is a plugin in WordPress, so WordPress needs to be loaded for s2Member to be active to then manage access. So a forum outside WordPress can't easily be protected with s2 (although not impossible, there have been hacks).
The easiest one that comes to mind would be bbPress, which also is a plugin and lives in WP. May not be the best forum software, but perhaps good enough for your needs.
The easiest way to manage access to it with s2Member, would be using the URI restriction. WP Admin > s2Member > Restrictions > URI
Thing is that the URI restriction works with levels, not ccaps.
You have two options that come to mind:
Remove the incremental access to the levels and use them as separate access capabilities, similar to ccaps. The problem with this approach is that the user can only have one level at a time, so they can't be added as ccaps are. https://s2member.com/kb-article/levels-wo-incremental-access/
Add some PHP conditionals to your forum templates, checking for the ccaps you want to require. WP Admin > s2Member > Scripting > Advanced PHP Conditionals
I hope that helps.