There's a long way to go before this is bonsai so there's likely to be more changes as it develops. It is good to make some early decisions but also be prepared to change as new developments appear.
It is always really difficult to give meaningful advice based just on a couple of 2D photos. We can't see any of the front/back bends or often where branches start and go to but I agree with removing the main trunk. It is tall, relatively straight, few side branches and has no taper. In short, nothing bonsai about it so it should go at some stage.
If you want some more increase in trunk thickness you might consider leaving it intact for a while longer but if you are ready to move forward just cut it now. Consider leaving some as dead wood. Dead branches are a part of many Aussie species. If you decide later that it's not right it is easy to chop then but much harder to put dead wood back if you chop right down straight away.
The lowest branch seems to have some good bends so maybe that's even a possible new trunk line?
