At present, templates work only for the Body of an email.
However, I often find myself sending reminder emails to groups I’m a member of (weekly events with Zoom links, for example) and it would be really useful if I could (optionally) set a particular default set of email addresses for the template (ideally both To and BCC fields) that would be added to the email I’m writing when I choose that template.
At present, the workaround is to remember the full list of email addresses I need to send this message to, or to find an old copy in the trash and copy&paste… Neater to include them in the template, I think!