I'm sorry there's not much traffic in the forums these days. It looks a substantial tree... If you have not already sought professional advice I would go to your local council and ask their arborists to take a look, or even the state DPI... University agriculturalists... would be a good study case and give you a range of opinions/options. Local authority would be first step I think as they would probably include this in tree preservation overview.

Maybe try a fungicide! Perhaps Triforine. Might help.