I have a fork here where I've begun to work on an email template editor.
You can edit the email templates using TinyMCE at Admin>Settings>Mail
jack-10000/itflow: Free and open-source web application for MSPs. Unifies IT documentation, ticketing, invoicing. (github.com)
Note, I have not added the subject lines, I will work on this soon.
I have also only added customer-facing email templates, not emails to admins/techs.
You can insert some variables into your template if they are applicable to the email's template type, such as [invoice_date] [company_name] [ticket_subject] [ticket_prefix] [ticket_number] etc.
There's a file called "db-email-templates.sql" that contains some sample email templates I have created.