I see we are in the same general area of the empire.
It's all a tradeoff between convenience, effectiveness and cost. What you have achieved so far is a pretty good result. (My Huawei 169 seems to be unhappy with any USB extension cable beyond about 1 metre).
If your USB stick has provision for an external antenna input, you could connect either a mag base mobile phone antenna for the band your ISP uses (mine uses the Optus network), or even a small yagi type antenna to get the best possible (but probably most expensive) option.
I've used the first option with good results up at Rawnsley Park to get a usable signal from the Hawker cell. It still involves a hike up the hill but that's what I'm usually there for anyway.
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PM me if you want to discuss specifics.
FoR