It looks like fields are added through the rwmb_meta_boxes filter before the post request as been parsed. So $post is not initialized.
Is there a way to add or modify fields after $post has been parsed/loaded? E.g. to modify the std, options, value or description?
Since every code (incl. custom database queries or API calls) in rwmb_meta_boxes runs whether it is needed or not it can slow down the site. If we could conditionally add them based on e.g. the queried post_type, it would speed it up.
It is not possible because the post object is initialized at a hook later than init. The filter hook rwmb_meta_boxes run at the init hook with priority 20. If you want to create some conditions before registering the meta box, you can use some WordPress built-in functions like get_option().
Or use the supported extension MB Include Exclude to load a meta box (field group) based on some condition https://docs.metabox.io/extensions/meta-box-include-exclude/