The brick sizes you shown are Australian i think, in the UK a brick is 215mm, 8 ft is a standard size which is 2438mm, you could fit a smaller door and increase the pillars to 2 1/2 brick giving more space inside the garage, depends on your car size too.
A 2 brick pillar will be 440mm, if you have planning for 3.3m make it that size otherwise you may regret it later, which would make your door 2420mm(just under 8ft).
For a bit more support you could also place a 5 or 6x2 across your trusses resting on top of the ceiling part, and under the sloping part, is the door going to open outwards? will it clear soffit/fascia.
If the ground was soft and you had access from both ends i would try raking it out with a narrow rake attached to a metal pole, would be a lot easier than taking out the wall, and how would you point up the replaced sections of brickwork?
You would probably need a cut off saw for some of the cuts anyway, so just get yourself a saw and not bother keep hiring a block splitter and a cut off saw when needed, you could always sell it on when finished.
Check with them regarding the bags, most merchants sell in bulk bags which weigh around 800kg, but still commonly called tonne bags, you may need to top up with 25kg bags.