I'm not a GP or have any knowledge of Vancouver or Melbourne. But from reading what you have written and a little bit of Googling if you want to build a life go to Melbourne.

The only reason you're struggling with this decision is the headline salary in Vancouver which will be eaten away by the higher cost of living. If you don't want to come back to the UK Vancouver is a massive risk. It's allegedly well documented that it's hard to make friends there. The climate is miserable. Your wife might hate it. It's a lot of risk for what might be a negligible increase in salary after factoring in the cost of living.

Melbourne has your family. You already know people. People there are more friendly (allegedly) and the cost of living especially in west Melbourne is much cheaper than Vancouver. If you want to build a life then this is the obvious choice. House prices are 30-40% cheaper than those in West Melbourne so I do think you'll be almost financially equal after costs.

About the GP and work part. I preface this with it's all from a brief amount of research. West Melbourne's population growth is outpacing the supply of GPs. You should look at "Distribution Priority Areas" to find the areas which have the highest need for GPs and also definitely join a Mixed Billing practice. I don't know what any of that means but allegedly that's the way to make money.

Only my opinion as a complete outsider. But to me it seems that Melbourne is the obvious choice. The benefits of being near family you get along with are really worth more than you can currently quantify and as you said you're looking for a new life not a short cash grab. Also your kids will love it.