I visited Australia a long time ago now and thought it such an average place. Honestly, it felt a bit backward. I'm sure it's changed now but I couldn't understand how anyone would choose to go and live there for the long-term. I found just under a week was enough and I was ready to leave. I found it boring, especially after visiting Asia. It seemed very plain vanilla and dull.
Don't get me wrong, the UK is certainly not perfect and the weather is shit much of the time. But I could never see the draw of Australia. It had little charm and little interest to me. Little culture, so far away from everything, little history and a very macho kind of undercurrent that permeated everything. Crocodile Dundee felt just a little too close to home. I was happy to leave. I'd never have considered it as a place to bring up my kids.
If you want quality of life, think France, Spain, Italy, Austria, Switzerland etc - even Eastern Europe would be much much better. Here you get what you know as a European, with the great education and culture, etc.
DH, if necessary he can spend another 3 years there and come back. If that's what he wants. As you say his life hasn't changed much. He still goes to work every day.
I wonder if the kids were out at childcare every day and you were working full-time if you'd feel more happy though. I think just spinning your wheels every day at home makes you more introspective and less likely to focus on the good stuff. You're not really progressing in life as your kids are your focus at the moment and you're devoted to them. There's no doubt that that's easier with family around because doing that alone is soul-destroying.
I don't think your DH quite understands what's going on.