Are you sure about it? Based on my accountant advice, just using your example, total cost of development of six units is calculated based on 6/8 of 2 million which is 1.5 million. Total income after selling cost of 100k is 2 million. So the company has made net profit of 500k! which will be taxed.
I have seen people buying the land in a company name with allocating preference shares to each investor. So after construction is finished, each get their nominated units as you suggested. I guess the only difference here would be two investors will be our family trusts and the rest would be the company who owns 6/8 of shares and register for GST and sells its nominated units on completion. A good lawyer should be able to draft a shareholder agreement for it.
I am not sure about your point regarding no tax if income from selling 6 units is equal to cost of building 8. I have been advised that tax office sees the profit equal to sale of 6 units minus (6/ of total cost although we intend to keep two units.