Thanks for reaching out to the Community! I understand how useful it would be to have a branded, master template for your surveys. Currently, we do not have the ability to save a template for survey invitations, but we are tracking customer requests for that feature. I will add your account to that tracking as another voice for that option. As a workaround, you could create a regular email template for that format, and replace the “Take this survey” link each time for the survey begin sent out.
If you have any further questions please feel free to reach out to us. We are always happy to assist you in any way we can!