I finally bit the bullet and replaced by 8yo battery. It was still ticking but figured I'd pushed it far enough.
I just went for whatever my local auto elec/battery man offered for as little as possible.
$165 for a Lion 485 red DIN77. About as cheap as it gets so hopefully lasts OK. 3yr warranty is good.
https://www.lionbatteries.com.au/pdf...nce%20Free.pdf