Redirect the Top Level User Page (parent page of all user pages) to a custom page (not the homepage, which is default for non-admins in the User Pages Add On).

my_pmproup_wp_parent_page_redirect.php

<?php
/**
 * Redirect the main user page parent to a custom page.
 */
function my_pmproup_wp_parent_page_redirect() {
	global $wpdb, $post, $current_user;	
	
	// Return early if the User Pages Add On is not active.
	if ( ! function_exists( 'pmproup_getOptions' ) ) {
		return;
	}

	$options = pmproup_getOptions();
	
	if ( ! is_admin() && ! empty($post) && ! empty($options['parent_page'] ) && $post->ID == $options['parent_page'] ) {
		if ( ! current_user_can( "manage_options" ) ) {
			$user_page_id = pmproup_get_page_for_user( $current_user->ID );
			if ( ! empty( $user_page_id ) ) {
				//redirect to the user's user page
				wp_redirect(get_permalink($user_page_id));
				exit;
			} else { 
				//redirect to a custom page
				wp_redirect( pmpro_url( 'levels' ) );
				exit;
			}
		}
	}
}
add_action( 'wp', 'my_pmproup_wp_parent_page_redirect', 9 );