I know you mentioned the documentation has changed to reflect this, but it hasn't. The documentation I've been looking at to work out how to resolve this issue indicates that the Design Template builder is still available (as you mention in your original question with the button showing on the screenshot). Very frustrating, I have spent a lot of time assuming it's something I'm doing wrong because it's visible in the documentation.

They really need to change it if they've removed the feature (No idea why though - the PDF builder is terrible for many types of businesses and the online quotes are much better - but unfortunately we have no control over what they look like). Thank you for posting this with the update, or else I would have still been trying to "fix" a problem that can't be solved haha

https://www.odoo.com/documentation/17.0/applications/sales/sales/send_quotations/get_paid_to_validate.html

Hello,

i checked again, the screenshot I see, is still without the button Design Template. Also in text there is no word any more mentioning this. They have now added in the part of the emails a new screen where there is also a green button to create template, but that is referring to the email template, not the quotation template for the portal users...