You're using FTTN so you may already be getting the maximum out of your service. Welcome to the technologically inferior and already outdated MTM.
Speed boost is not a guaranteed speed, it is up to 100/40. If you are not getting this you can cancel it or opt for the up to 50/20 version
I don't see anything wrong with your line stats, theoretically with that downstream attenuation higher speeds (but certainly not 100/40) may be achievable, but that would be based on copper wire gauge, bridge taps, crosstalk, etc, which can't be determined from modem stats alone so I wouldn't get your hopes up and that cant be used as a basis for argument. Your maximum line rate is generally a good indicator of highest achieveable speed. For what I can see there is not enough grounds for Telstra to lodge a fault with NBN™. VDSL is a lot more finicky then ADSL and does have a exponential drop in speed over distance, unlike ADSL which is more linear and robust to other influences
You can Contact 1800 TFIBRE and they can run some diagnostics on the service, and you can speak to them about dropouts, however if everything looks ok on their end there is not much else that can be done.