Hi, is it possible to store MB own settings as MetaBox CustomFields or MetaBox CustomFieldsGroups etc not to "wp_posts + wp_postmeta" table, but to a custom DB table?
Why? Because MetaBox and CustomFields is NOT the Blog Posts for sure!
Currently, it's not possible. Each field group is a post and will be saved to the table wp_posts as a post type meta-box. It works like other plugins like WooCommerce: order, coupon.
If you don't want to use the field group with UI, you can use the code to register custom fields. Refer to the documentation https://docs.metabox.io/creating-fields-with-code/