RSS: fetch_feed caching with different caching delay per feed?

You can use the second parameter passed to the wp_feed_cache_transient_lifetime filter which, in the context of fetch_feed(), is the URL of the feed:

add_filter( 'wp_feed_cache_transient_lifetime', function ( $time, $url ) {
    if ( $url === 'https://example.com/rss' )  {
        $time = MONTH_IN_SECONDS;
    }

    return $time;
}, 10, 2 );