Back after a break, I ran out of steam trying to make all this work.
And I'm pretty much there again now.
I fitted a Pi TV Hat to the Pi to try to eliminate any chance of odd gears causing a problem. Swapped over the TV coax cables to find what seemed to be the best one. Rebooted the Pi many times.
Maybe a tiny bit closer but I still have no idea where to go from here. So I've recorded a video of the TVHeadend setup and in it you can see the scanning progress bar quickly go from 0 to 37% and then stay there from about the 30 second mark to about the 1:05 minute mark, just did that to show that the scanning stops there and doesn't progress any further, even if I leave it for half an hour or so.
Lemme know if I'm doing anything wrong please.
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You're still using the pre-defined muxes in the set-up wizard. They're out of date. Don't use this method for set-up.
You need to add your muxes manually as I explained previously.
I always bail out of the set-up wizard after setting English as my language, and then take over manually. (Keeping the frequency tables up to date for every country around the world has not been something the small number of TV Headend devs have been able to do - and users aren't updating it with pull requests to github either)
Because you've got a tonne of rogue stuff in your networks and muxes tables I'd start from scratch or at least delete the stuff you've already added. (You added repeat networks which is why your network and mux view is so busy)
In the configuration-DVB inputs-network tab delete all the networks that are currently there, and add a new DVB-T network (This 'network' is a TV platform - nothing to do with computer network). You want to create a new DVB-T network with a name of your chosing - e.g. OTA, Terrestrial, Freeview etc. You don't need a different network for ABC, Seven, Ten, Nine etc.
In the configuration-DVB inputs-muxes tab if there are still existing muxes then delete them (though deleting the networks should have deleted the muxes associated with them) and add new ones with the right frequencies to the network you created in the steb above. (The post I made earlier has the current frequencies for the area you live in I think - they're different to the ones you are adding automatically with the out of date channel list the set-up is using).
Note that you need to use the frequency in Hz - so ABC on 620.5MHz = 620500000. You need to select delivery system as DVB-T, and probably need to select bandwidth as 7MHz. You may also need to set constellation as QAM/64, Transmision Mode as 8k, and possibly Guard Interval as 1/16 (I've NEVER had to set Guard Interval manually) - though often leaving everything but delivery system, frequency and bandwidth to Auto works fine with most tuners
In the configuration-DVB inputs-TV adaptors tab - add your network to your DVB adaptor and enable it.
Select your network in configuration-DVB inputs-network and click 'Force Scan' to force it to tune the frequencies/muxes you've manually added. (It may already be scanning - but it doesn't help to kick it into doing it)
In the configuration-DVB inputs-muxes tab you'll see whether it's OK (tuned and found services) or Failed (Pending means it hasn't tuned yet)
In the configuration-DVB inputs-services tab you'll see the tuned TV channels. Click 'Map services' to map them to the Logical Chanel Number that Australia uses for each channel (i.e. the channel numbers everyone in Aus uses - ABC on 2, SBS on 3, Ten on 1, Nine on 9 etc.). In Kodi you'll also need to enable using channel numbers in the Live TV Settings in Kodi System settings - if you want standard channel numbers in Kodi.
