In case it helps someone else:
For the time being, I added a WP role “Community Member”, linked it to the membership purchase, then added this code to functions so the directory only shows users with this role:

 /**
 * Exclude Users from BuddyPress Members List unless they have specific WordPress role.
 *
 * @param array $args args.
 *
 * @return array
 */
function buddydev_exclude_users_by_role( $args ) {
    // do not exclude in admin.
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
        return $args;
    }
 
    $excluded = isset( $args['exclude'] ) ? $args['exclude'] : array();
 
    if ( ! is_array( $excluded ) ) {
        $excluded = explode( ',', $excluded );
    }
 
    $role     = 'community_member';// change to the role to be shown.
    $user_ids = get_users( array( 'role__not_in' => $role, 'fields' => 'ID' ) );
 
    $excluded = array_merge( $excluded, $user_ids );
 
    $args['exclude'] = $excluded;
 
    return $args;
}
 
add_filter( 'bp_after_has_members_parse_args', 'buddydev_exclude_users_by_role' ); 

Thanks Brajesh, that makes sense!