There are some OTC desks in Australia that can assist in selling large sums of crypto. I have used Finder in the past for clients.
Avoid tax I am a tax accountant in Aus. Best way is to minimise tax is to offset gains with losses (said above) and use the 50% discount method, available for assets held > 12 months. Other ways are usually illegal.
Depending on how much you are setup to make from the sale, it may be worth trying to immigrate to a tax haven country, if only for a year or two to cash out. Most of these countries only require you to be a resident for a year before you can take advantage of their favorable tax policies.
For example, in the US, we have the ability to “immigrate” to Puerto Rico, and by immigrate, I really just mean move because it is a US territory. Puerto Rico offers massive tax advantages to people that move there and live there for some period of time, which I don’t remember off hand. They also offer massive tax benefits to people that stay there and start a business employing locals.
I’m sure there are similar options for Australians as well, that was just one example I know of off hand since that’s my gameplan if MAID/SAFE ever moons.