I have not looked into the entire grid. I am only specifically concerned about South Australia, and I see no reason for concern in the next two or three decades based on current trends, technology and assets. Trying to guess what will be the right solutions in 20-30 years time would be like trying to guess in the 1980s or 1990s when the Northern Power Station was new and there were no wind farms in SA that we would be where we are now.
New South Wales clearly needs something more than it has now, but I haven't looked into whether that is a new coal or gas-fired power station or the large solar farms that are about to be built near Balranald and some wind farms somewhere, or a new interconnector from South Australia (Hornsdale wind farm output is contracted to the ACT government!). NSW has the Snowy Mountains Scheme which includes capacity for some large pumped hydro schemes, provided the water isn't needed downstream. The reason an interconnector is not a trivial fix is that SA and NSW both have grids designed for the generation to be central, and the transmission lines to near the border become progressively thinner. To install a new large source or sink of electricity requires significant enlargement of the network to connect it.
I agree a large nuclear power station built then could have been a good idea, but I suspect we are past that now, even if it was possible. I still wonder if fully-contained units like in nuclear submarines or ships would be useful as power stations in outback communities that are a long way from the national grid. When they wear out, a new one could be trucked in and hte old one taken out for reprocessing.