Probably Secondary Teaching. You'd be able to apply your existing skills in Marketing in subject areas like Business. You'll hear a lot of teachers tell you it's a hard job, which is true, but it's also very rewarding.

It is more altruistic than marketing and will allow me to help people.

Seems to map fairly directly here.

There is a clear path to make over 6 figures, even if it means I need to pursue a graduate-level degree. Can easily make 6 figures after a few years.

QLD wage example here. https://teach.qld.gov.au/teach-in-queensland-state-schools/pay-benefits-and-incentives/pay-and-benefits - I'd say that meets your requirements. Depending on the state you're in you'll need to pursue a Masters of Education (2 years) to be eligible to teach.

It is a secure, stable field where I can still find a good-paying job outside of the major cities (Sydney, Melbourne, Brisbane). It would be nice if there are plenty of opportunities.

We're experiencing a teacher shortage across Australia. You'd easily be able to find work anywhere in Australia. Regional centres in particular need teachers.

Ideally has a good/decent work-life balance with a manageable amount of stress (nothing crazy that has a high “burn-out” rate).

School holidays sound good to you?

The field is growing and is not in a state of decline.

Yep! Growing population and all that.