I opened my account at the Bangkok Bank (Kao San Road branch) when I was still on a tourist visa. When I mentioned that my intention was to retire in Thailand, they immediately accepted.
A big difference : the 800k remain YOURS, while the million for the Elite Visa is lost for ever... If, after some time, you decide to leave, you take your money back (with some profit, given the increase of the value of the bath) ! ;-)
As a general rule, any country that bans its citizens/legal residents to travel outside, does not prevent its citizens/legal residents who are already outside the country at the time of the ban, to fly home. They "just" might ask you to self-quarantine. The thing is, the more you wait, the less flights there might be from Thailand to Australia (airlines will not fly with empty planes, probably very soon).