Multisite Subfolders Countries Languages

Hello there,
I consider using multisite network for my gTLD .com domain.
After I checked the best options (subdomains or subfolders or ccTLDs) I will go for subfolders.
Before I set up the network and start adding new websites to it, I have one question I could not find an answer to.
If I target different English-speaking countries (for example Australia and Canada, USA etc.) what will be the best setup for the domain?
http://www.domain.com/au or
http://www.domain.com/en-au?
Hreflang will be used later but my question refers to the best domain setup itself.
Would it be enough to just use the country code “au” in the example or would it be better to use country and language “en-au”?
Thank you very much in advance

Qu.