wp rewrite and custom RSS feed

I created a custom feed template “testfeed” which I want to use for categories e.g.: https://www.test.com/category-1/testfeed

The problem I have is with wp_rewrite, I cant get it working:

function custom_feed_rewrite($wp_rewrite) {
	global $wp_rewrite;

	$feed_rules = array(
		'testfeed/(.+)' => 'index.php?feed=' . $wp_rewrite->preg_index(1)
	);
	$wp_rewrite->rules = $feed_rules + $wp_rewrite->rules;

    return $wp_rewrite;
}
add_filter('generate_rewrite_rules', 'custom_feed_rewrite');

This works https://www.test.com/category-1/?feed=testfeed but I want to have nice URL’s.

Any help would be appreciated.

Thanks!