ok – so this isn’t what I wanted but it does the trick.
Adding a function to redirect when the user hits any term but one:
function my_page_template_redirect() {
if( is_singular( 'custom-post-type' ) && !(has_term('term', 'custom-taxonomy')) ) {
wp_redirect( home_url(), 301 );
exit();
}
}
add_action( 'template_redirect', 'my_page_template_redirect' );
Answer based on this one: Disable Single Post View for Specific Taxonomy on Custom Post Type