After much searching, I found that Alpine make a bespoke kit just for this car. The G6/G7 bit of the product name indicates which mark of Golf it is for, although the electronic components are very similar.
It supports Android Auto and Apple Car Play, has DAB built in (more on that below), can play USB music with album artwork displayed, the steering wheel controls still work, as does the dash display. The car's built in microphone works (although it was very difficult to work out how - had to get Alpine tech support to assist). Installation is fiddly and you need to cut small parts of the fascia away (reversible by replacing a couple of parts) so I'd recommend getting it professionally fitted if you don't know what you're doing.
I also purchased the KAE-DAB1G6 DAB Radio Signal Splitter - this thing is ingenious. Its invisibly installed into the rear spoiler, and uses the car's original amplified antenna, to provide a very high quality DAB signal to the head unit (my original stereo in the car did not have DAB).
Its been tricky to learn to use, it was very expensive but 2 months on I've got the hang of it and am very happy with it, and think it was a worthwhile addition to the car.