I researched a similar question in 2019. I remember that there was an upper age limit for the cargo ship option. Children were completely impossible, so I abandoned the plan and didn't research any further. I seem to remember the age limit was pretty low, maybe 60? Depends on the company of course, and it might have changed. I think it's because there's only minimal medical support on a cargo ship, and the risk of e.g. a heart attack rises with age.
There are around-the-world cruises that stop in Australia and in Europe. That's a completely different price range though!
I guess you could find cruises from Australia to South America. There's definitely lots between South America and Europe. Maybe that's less expensive?