@ricsca you can edit email models.
You will found the template that can be customized in the operator Dashboard.
I also use {%ticket.threadMessage%} in that template.. and I'm sorry I cannot understand well your request. Maybe a screenshot of what you get, what is wrong, an example on what is expected should help to help you better.
If you wanna have just a partial reply text, this is not supported.
You can have full reply or remove the {%ticket.threadMessage%} and ask the client to login in the web interface.