New application

It depends. If your situation is straight forward and you're willing to take the time to do the research, then it's fairly easy to do it yourself.

Ali29 said:

If we start by applying offshore (309/100) then he comes to Australia and stays, can we change the visa over to an 820/801or does it have to be a brand new application?

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You can't change it. If you apply for the 309/100, there's no bridging visa so he'd need a valid visa to come to Australia if the 309 hasn't been granted yet. If he wants to make an onshore application, that means doing it all over again and paying a new fee.

Ali29 said:

Thank you.

I am not sure which Visa to apply for: 309, 820 or PMV. Which ones allows him to work AND get Medicare?

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300 = Full work rights, no Medicare until 820/801 lodged
309 = Full work rights, Medicare
820 = Full work rights, Medicare

If you're asking about work rights and Medicare while waiting for a decision, then that's the onshore 820/801. He needs to get onshore on a valid visa without the no further stay condition to be able to apply. Once applied, a BVA is automatically granted but isn't in effect until the current visa he's on ceases. So unless the visa he enters on has work rights, he won't be able to work until the BVA goes into effect or the 820 is granted, whichever is first.

Not sure what you're asking. The 300 is an offshore visa. Once granted, he can go to and from Oz as much as he likes within the 9 month period. But once the 309 or 820 are granted he can go in and out freely as well.

If the requirements can be met for a 309/100 or 820/801 then there's no use in considering the 300 as an option.

The question then is whether or not he wants to apply onshore or offshore.

Yes, there are restrictions and a BVB would need to be applied for and granted to travel. But the ETA isn't meant for long term stays either so unless he's only visiting every once in a while on the ETA, then don't expect to be able to spend a lot of that wait time onshore with it.

You can explore other options. For example, the 462 if he's eligible (You mentioned USA so I'm thinking he has a U.S. passport?).