You can create multiple report templates and assign a default from within your firm settings.
Ensure you are in your Firm account (menu bar in the top left). You'll be able to see more settings from the dropdown menu. You can adjust templates such as emails, reports etc...
Clone the original Clarity template and make adjustments accordingly. Then reassign as a default.
Click into the Report menu to use it for a client. (Ensure you're inside a client file).
When sending the report you'll be able to change the default template if needed but as you'll see below; the newly created template is now the default.