The PCM5102a is supported quite well (and quite easily) by Raspbian. It requires enabling i2s and dtoverlay=hifiberry-dac but it fails on the standard “Gorgona Omega” SD-image. I wonder if it relates to the GPIO18 conflict you reference (whatever that is). GPIO18 is the bit-clock hence required by the PCM5120a. Could it be that Zynthian uses GPIO18 for another purpose and hence hijacks the pin from the i2s driver?
I2S is quite simple: PCM data is streamed serially on the data line and is clocked into the ADC using the bit-clock. ADC switching (left / right - or even more channels is permissible) is done by the LRCLK. A master clock can drive the ADCs but tying this input to GND makes the PCM5102a generate it internally which is why this chip works with RPi which does not proved a master clock.
[Edit] I am not sure why but it is now working:
Installed “Gorgona Omega” SD-image.
In Web interface:
Set audio interface to HifiBerry DAC+ light
Set display to Generic HDMI display
Set wiring to Dummies
WORKING with display on HDMI and USB mouse pointer. Sound from I2S. USB MIDI input.
It occurred to me after posting this that GPIO18 can’t be relevant because it is used by HiFiBerry DAC+ Light.