Commerce email receipt not sent after order, but manual Send Email in Order screen works
Simple step missing on the setup of this. I needed to actually assign this email to the status New, to send when a New order is placed, in the Order Statuses section, as per this documentation. So simply a case of RTFM:
https://craftcms.com/docs/commerce/3.x/order-status-emails.html#selecting-an-email