GMB for multiple locations in a country but my business is remote / at the home

[apextuition]

I run a tutoring business in Australia. We started off in Melbourne (we also have a physical office here, but no business is conducted from here).​

We now provide services in Sydney, Brisbane, Perth and Adelaide. Our tutoring is not done at a location but where the client is.
Our current GMB is a service area business including all the five cities but this feels sub-optimal. It ranks well in Melbourne but non-existant elsewhere
Should I set up a GMB for each city? Can the phone number be the same for each GMB? We have one number that runs the business nationally. What about the address? We only have one address which is our office, can this be re-used for the others? Or do you need a unique address? On our website we only show our office.
Or, can I better optimise my one current listing to be nationwide?
Does anyone have advice? I'd be more than happy to pay for someone's time if they have dealt with this situation before.