"So what exactly has Australia to gain from AUKUS? I still fail to see there benefits."
SSN's can operate longer and safer further N than the diesel boats - unless you look at a map its hard to believe but China is actually closer to Finland than to Australia
SSN's are hard to detect and can carry a serious conventional punch - they last a long time and, overall, are very flexible. It was SSN's that kept he Argentinean navy in port thru the Falklands War