Build one. You won't find a perfect rig for anywhere near that price. From all the rigs I've tried out, the only good ones are the stupidly expensive ones. The cheap ones (which are anything but cheap, some of them cost close to 1000 Aussie Dollars) all have flaws. Almost all of them have a significant amount of flex, which some people can put up with, but some people can't. I find the prices of these rigs almost insulting they're so expensive for what you get. Basically, $800 - $1000 AUD will get you a cheap chinese racing seat knock off, bolted to $20 worth of steel that has been bolted together in ill-fitting segments. A year two apprentice metal worker would do a better job in a couple of hours.
I almost spent $800 on a rig last year, then I got to test one, and immediately changed my mind. So glad I decided to rebuild my rig and make it better. It's cost me next to nothing, lasted for 4 years so far, and with the recent rebuild and reinforcement that I did, it'll last as long as I need it to, and would put up with DD wheel torque without flexing at all.
Find/buy a seat, and the rest of the rig would cost $20 - $50 tops to build.
Edit: This assumes you're handy with a welder and grinder. If not, you'd presumably have a mate who is who could help.
If I were having the issues you're having, I'd send the wheel back to Fanatec. It shouldn't do that, ever. I've had my CSL-E for a while now, and it's never lost it's centre. I've seen people reporting having trouble losing centre on Fanatec wheels in the past, and it was always an issue that required sending the wheel back to Fanatec.