What is the point of a draft email if you lose it after sending it and have to create it again? The whole concept of Mozilla Thunderbird's draft email makes no sense, because Thunderbird makes a lot of work for the user if the user wants to send an email to thousands of recipients without using BCC, because BCC is unfortunately recognized as spam because the recipient is not in the recipient field.
The "edit as new" function is unfortunately not a good alternative to the "send and keep draft" function suggested here, because the recipient field is unfortunately not empty when you "edit as new", which also costs time when you consider that you have to empty the field thousands of times for thousands of emails.
Newsletter software is also not a good alternative, because the emails they send are often recognized as spam.
That's why I'm suggesting the "send and keep draft" function here.