ok, I’ve been kicking this around for the last 3 hours!!
Capabilities are spread all over the files in bbpress, so it’s taken a while to get to something.
But I’ve learnt some good stuff along the way !!
This is close to a perfect solution, and seems workable
add_filter ('bbp_register_forum_post_type', 'rew_change') ; 
function rew_change () {
	$rew = array(
				'labels'              => bbp_get_forum_post_type_labels(),
				'rewrite'             => bbp_get_forum_post_type_rewrite(),
				'supports'            => bbp_get_forum_post_type_supports(),
				'description'         => __( 'bbPress Forums', 'bbpress' ),
				'capabilities'        => bbp_get_topic_caps(),
				'capability_type'     => array( 'forum', 'forums' ),
				'menu_position'       => 555555,
				'has_archive'         => bbp_get_root_slug(),
				'exclude_from_search' => true,
				'show_in_nav_menus'   => true,
				'public'              => true,
				'show_ui'             => current_user_can( 'moderate' ),
				'can_export'          => true,
				'hierarchical'        => false,
				'query_var'           => true,
				'menu_icon'           => ''
			) ;
	return apply_filters( 'rew_change', $rew );
}
add_filter( 'bbp_get_caps_for_role', 'rew_moderator', 10, 2);
function rew_moderator ($caps, $role) {
	if ($role == bbp_get_moderator_role() ) {
		$caps['edit_others_forums'] = true ;
		$caps['delete_forums'] = true ;
		$caps['delete_others_forums'] = true ;
		$caps['publish_forums'] = true ;
		$caps['read_private_forums'] = true ;
		$caps['read_hidden_forums'] = true ;
		$caps['edit_forums'] = true ;
				
	}
return $caps ;
	
}
The line
'capabilities'        => bbp_get_topic_caps(),
should say
'capabilities'        => bbp_get_forum_caps(),
and my 2nd filter should then make that work, but for whatever reason it doesn’t.  
But as it gives the topic permissions which for a moderator are what you want for forums, it should be fine.