Can I use both a custom excerpt and a trimmed excerpt?

We can try to filter the excerpt through the get_the_excerpt filter. By default, if we have a manual excerpt, the manual excerpt will be used and not the automatically created excerpt which is created by the wp_trim_excerpt() function.

We can alter this behavior. What we will do is, when we are inside the main query (in_the_loop()), we will return the output from the wp_trim_excerpt() function. This way, we keep all filters as per default. Whenever we are outside the main query, we can return the manually created excerpt, if it exists, otherwise the normal excerpt

add_filter( 'get_the_excerpt', function( $text )
{
    if ( in_the_loop() ) 
        return wp_trim_excerpt();

    return $text;
});