Answering my own question, I’ll add this in case it’s of use to someone.
It seems to work ok with…
// This delivers valid feeds, with the correct templates.
remove_all_actions( 'do_feed_rss2' );
add_action( 'do_feed_rss2', function() {
$rss_template = get_template_directory() . '/feeds/item-feed.php';
$rss_template2 = get_template_directory() . '/feeds/notes-feed.php';
//if ( $post_type="item" )
if( get_query_var( 'post_type' ) == 'item' and file_exists($rss_template ) )
load_template($rss_template);
elseif ( $post_type="post" )
load_template($rss_template2);
else
do_feed_rss2(); // Call default function
}, 10, 1 );
This enables use of a custom template for the feed of normal posts, and use of a different custom template for the feed of the CPT ‘item’.
The feeds differ in channel title/link/description.