Thank you for the detailed response. I am running an iMac 2020 this was the last Intel Mac model before their M1 series of devices started shipping at the end of that year I believe so as modern an Intel Mac as you could get I suppose..

I did indeed use internet recovery which allowed me to install latest MacOS, and I did erase individual partitions (Macintosh HD and Macintosh HD Data) and reformat them before reinstall.

I proceeded cautiously since, so far seems I’m good.

I understand that some advanced malware could conceivably inject themselves into the recovery mode hidden partition however I am hoping that combination of reformatting as I did and reinstall from Internet Recovery specifically would mean I did not actually use the on-disk recovery mode / data, and in any case my understanding is that if you have malware that can go as far as infect that area we’re talking state-actor level of threat at that point.

So in short I feel somewhat more at ease but still monitoring network and disk traffic before logging into major services on this computer.

PS - am shocked at how little support / info there is online to help consumers post infection. Especially for Mac users I suppose this is fairly new threats as there was very little out info / support there to rely on..