So, in the end I did email Schut Car Systems in Holland about this because I thought they may get an eventual customer out of it, even though I can't travel over now. Plus, the information might be useful to others on here. I asked them about installing Set 5 from
this link and this is what they said -
"Set 5 cannot be installed in your car. At least not the way we have it on our website. We need to add an amplifier.
For the rear speaker output on the radio to work (and activate Fader functionality) the car/radio needs to be coded. The ID3/ID4 and Golf 8 etc. are however on a new platform where coding needs to be done online. This is not something we do ourselves and to avoid problems in the future when the car for instance receives a firmware update over the air and for any reason resets coding we work around this.
We use the front speaker outputs to supply a 4-channel amplifier with signal and power new front and rear speakers. This not only adds rear speakers but also is a drastic audio upgrade.
Only thing is that you don't have the ability to adjust the balance of the speakers between font and rear through the radio (Fader). We can do this for you and show you how you can do it yourself.
The centre speaker is unchanged since this speaker is also used for E-call functionality.
Such an upgrade would cost € 1099.- completely installed."
This seems fair enough to me I think. It would basically take VW's implementation and make it 'old school' again by having a normal amp supply separate front and rear speakers. Expensive way to add rear speakers, I admit, but if you look at it as a general all around audio improvement to make it sound even better than cars with the factory rear speakers then I could maybe justify it to myself.
Doing it this way means I wouldn't have to worry about future VW software or firmware updates hobbling my system by undoing the re-coding.
I suppose though, at the point you're just literally taking the two-channel output from the head unit, putting it into a 4-channel amp and running that to speakers, most UK-based car audio specialists would be able to do that for me too. But I'm still tempted to wait until international travel resumes and go to Holland because a) they sound familiar with the ID.3 so are likely to know what they're doing, b) they seem like a nice friendly company, and c) I quite like the idea of taking the ID.3 into Europe on holiday anyway
