Hi there, Olly. The email subject "Your estimate is ready" is set by default and can’t be changed. As a workaround, you can download your estimate, save it as a PDF, send it to your own email, and then forward it to your customer.
In QuickBooks Online, you can only change the names of the templates in the Custom Form Styles page. Go to the Form section and change Estimate to Qoute in the name field.

After that, download the form.

This is what it looks like:

Let us know if you have further concerns.