That is a pretty wild rack, and a lot of gear. The motorcycle case sits awfully far back on your rack, and undoubtedly adds stress to the rack.

Perhaps it is time to start looking at a cargo bike build.

The XtraCycle FreeRadical is an interesting add-on for an existing bike. I think there was someone on the board that posted recently about one.
Xtracycle Longbike Cargobike kit
Xtracycle Free Radical

Ahhh, you're in Australia. Shipping would be fun

But, it may give some ideas.

I welded my (first) cargo bike together from pieces. It came out to be a real beast. But I'd probably try to lighten it some if I did it again. Still, it is a beast.