So a few more hours of head scratching, and it sounds like a common problem…
Apparently the Picade X Hat doesn’t have a hardware mixer so volume is supposed to work via a software mixer: Picade volume stuck at high - RetroPie Forum
Creating a new softvol device via .asoundrc Alsa Opensrc Org - Independent ALSA and linux audio support site meant alsamixer now had a volume control for the card (snd_rpi_hifiberry_dac). Setting this volume then also meant amixer outputs with the new volume details.
The last piece of the fix was then setting the Audio Device in Emulation Station to use the newly created one from .asoundrc (Master in my case). Volume now working with RetroPie 4.8!