Hi Hedo,
I have the exact same issue, so feel your pain.
I suggest you first call technical support again, who will run a few tests, shrug, may send a technician out, which is pointless because the line is obviously fine if you can get full speed in off peak times. In my experience I have not seen Techincal support have a solution to local congestion.
If you are not happy with the solution offered, make an official complaint to Telstra. Then if you are not happy with the response from the Complaints process, you may lodge a complaint with the Office of the Telstra CEO or if you believe you have a strong case then lodge a complaint with the Telecommunications Industry Ombudsman.
How to make a complaint to Telstra https://say.telstra.com.au/customer/general/forms/Email-Complaint
Contact the Office of the Telstra CEO https://service.telstra.com.au/customer/general/forms/contact-ceo-form
Submit a complaint to the Telecommunications Industry Ombudsman https://www.tioonline.com.au/consumers/new/
Unfortunately local congestion seems to be systemic across large parts of the Telstra network at this time. Telstra seems to think local congestion is acceptable. My theory is that any area that is slated to be part of the HFC NBN network is not having any upgrades at this time, because Telstra does not want to spend the money upgrading the network when NBNco will do it for them later. So basically they save money and make a larger profit at the expense of their customers. They do continue to sell the current network to new customers although it does not have the capacity to handle them.