If you use(d) the audio out function on Airport devices to add speakers to iTunes, take a look at Volumio[1]. A Raspberry Pi is an ideal Volumio host, configuration is very straightforward and the results are excellent.

[1] https://volumio.org

e.g. https://github.com/volumio/shairport-sync compared to https://github.com/mikebrady/shairport-sync

But thank you - this lead to finding https://github.com/badaix/snapcast