What part of the aplay output are you actually interested in?
card 0: Audio [Fe-Pi Audio], device 0: Fe-Pi HiFi sgtl5000-0 [Fe-Pi HiFi sgtl5000-0]
According to ALSA terminology, this would be
card = 0;
card_id = "Audio";
card_name = "Fe-Pi Audio";
device = 0;
device_id = "Fe-Pi HiFi sgtl5000-0";
device_name = "Fe-Pi HiFi sgtl5000-0";
The terminology here is a bit confusing (for me), so I was under the impression you wanted the card name, but it looks like you are interested in the card_id?
Looking at the dacs.json entry, this is mapped as:
{
"id":"fe-pi-audio",
"name":"Fe-Pi Audio", // ALSA card_name
"overlay":"fe-pi-audio",
"alsanum":"2", // ALSA_card
"alsacard":"Audio", // ALSA card_id
"mixer":"PCM",
"modules":"",
"script":"",
"needsreboot":"yes"
},