Customising WC System Emails

I have been trying to word my system email (order completed etc.) how I like. I have been to WC>Settings>Emails, and I can change the subject, heading etc. and add text. The problem is, the text only appears at the end of the email and the text WC puts at the top isn’t what I want. How can I change it?

The email is set out as below:

Heading: Thanks for shopping with us

WC Text: Hi Lindsay,

We have finished processing your order.

Order Details: [Order #28968] (June 13, 2023)

Email text: I can add what I like here

I want my text to appear where the generic WC text is.