I certainly am one to try to use what I already own, but sometimes that plan is not the best. I think you need a different rack.
Of the Tara and Ergo (both Tubus), I own both of them and I think the Ergo works a bit better with Ortliebs. If you have panniers other than Ortlieb, I have no comment on which would be better. I just checked, it appears the Ergo is no longer in production but might be available at some retailers.
I do not have an Audax, so I am not sure how the brakes are fitted. Are there cantilever brakes on the front of the fork? If so, if you can find a Tubus Smarti rack, that might work great. I believe the Smarti is no longer in production, so it might be hard to find. SJS does not list it on their site, but google shows it is out there elsewhere.