Function.php WordPress used to work - REDIRECT 404 to Parent CAT

Hi.

I have this code in FUNCTIONS.php that detects a 404 error and REDIRECTS (with a 301) to the parent CATEGORY

It still works - BUT - if the parent is ROOT (home) it redirects it there too ... I want 404 error to show if it's a genuine 404 like this https://chronodivers.com/thisisrubbish

For anything else I want it to redirect to the PARENT CATEGORY - as this does https://chronodivers.com/omega/omega-seamaster-planet-ocean-watch-sales/thisisRUBBISH

The code used is as follows

/**
* Add REDIRECT to PARENT
*/

function __404_template_redirect()
{
if( is_404() )
{
$req = $_SERVER['REQUEST_URI'];

if ( is_file( $req )) {
return; // don't reduce perf by redirecting files to home url
}

// pull the parent directory and convert to site url
$base_dir = dirname( $req );
$parent_url = site_url( $base_dir );

// redirect to parent directory
wp_redirect( $parent_url, 301 );
exit();
}
}

add_action( 'template_redirect', '__404_template_redirect' );

I am happy to edit the functions.php file with the correct code

Cheers

Tristan