Thank you to all contributors for these helpful & timesaving pointers.
I thought to post a couple little additions of my own:
* Before selecting the final ‘Add Blog’ button in You Tube, ensure you’ve logged out of WordPress (otherwise it just whirs round and round doing nothing).
* If anyone’s trying to do this who has a static WordPress homepage, rather than a blog WordPress homepage, I confirm it’s is still the root domain that YouTube is after for the API URL, ie: use https://example.com/xmlrpc.php, not https://example.com/my-blog-contents-page/xmlrpc.php. Or at least, that’s what finally worked for me!