I suggest to allow adding unlimited number of email templates. On the email compose page, add a dropdown to select a template to use. And in Settings, select default template for each type of document.
Currently, Manager.io only allows 1 email template per document type. But my business send different sales invoice emails for retail customers and wholesale customers. And we also send monthly recurring invoices for equipment rental. So we need 3 invoice templates
What I do is make 3 templates and save them as text files. Whenever I need to change the invoice email type, I copy from text file to the Email Template in Settings. And depends on the kind of customer, I also send different quotation emails.
It would be useful to save more email templates and quickly switch among them.