How can I use a template to reply to an email?

See https://addons.thunderbird.net/en-us/thunderbird/addon/simple-template/

Thank you for your help!

Unfortunately, I have an account that I use for both private and professional purposes. If I understand “simple template” correctly, it is not possible to choose between private and professional use. Is that right?

Yes, a single template is defined for all accounts or separate ones for each account. In your case, an add-on like Quicktext or StandardTexter might be more useful.