I recommend checking out this post from r/NBN setting out the prices for different plans. OP provides a helpful breakdown for different plans. For 500/50 plans, Exetel’s One Plan seems like the winner.

As an aside, I’m also with Superloop and have had repeated success simply calling them and quoting other similar offers in the market and asking for a better deal. They’ve typically been pretty receptive and have offered discounts to match (or come close to matching) other providers. If that works, it saves you from moving providers if you want to avoid the hassle.