I would like to store forum threads and replies in a custom table using Metabox, MB Custom Table and MB Frontend Submission where forum posts and replies are contained in a custom table (not wp_posts).
I do not wish to use BBPress (or suchlike) because they are overly complex for a simple forum and need to keep wp_posts segregated for page/post/cpt content only.
Hello David, at the moment, the MB Custom Table still relies on the custom post types to store the post data such as post title and post content. It supports only saving custom fields in the custom tables.
We're working on an enhancement that brings more power to this extension, which allows you to store any content separately from the wp_posts table. That will include both the custom fields and post fields. But it's not ready yet and you need to wait.