Just as an update, since the ECU has been replaced, it's been perfect.
On the topic of the ignition barrel, it was starting to do EXACTLY what my old VZ did back in the day before it seized up on me, so whilst it was still somewhat working, I did get the ignition barrel done by a local locksmith, $400 for a replacement barrel + brand new key blade, which is perfect because now I can get rid of my bad copy of a worn down key and get a brand spanker cut. From memory, that wasn't much more expensive than what it cost to get done by Holden back in '09.
We did also discover during the barrel replacement that the ignition switch was held on with glue and a cable tie, probably due to aforementioned "hotwiring" to make the bastard start due to the dodgy ECU.
I'll get that replaced, as since the barrel has been replaced I suspect the ignition switch dodginess is responsible for the cluster rebooting a couple of times when I went over a bump.
Either way... happy as larry for now. I'd almost use this as my daily driver, if fuel wasn't $2.20 a litre (she's thirsty!), and my daily at the moment is an EV lmao.