You can try the following to modify the links of the menu items that are pages:
add_filter( 'nav_menu_link_attributes', function( $atts, $item, $args, $depth )
{
if(
isset( $item->object )
&& 'page' === $item->object
&& isset( $args->theme_location )
&& 'primary' === $args->theme_location
)
$atts['href'] = home_url(
sprintf( '/#/page/%d', $item->object_id )
);
return $atts;
}, 10, 4 );
where we target the primary theme location.
This should give this kind of links:
http://example.tld/#/page/12345