Hello from 2024 
I am in the same boat. I want to capture only traffic on qmimux0 interface. When I execute
uci set darkstat.@darkstat[0].interface='qmimux0'
And restart darkstat, it fails to work. But, running manually from the console
darkstat -i qmimux0
Works fine. What can be done?
edit: I don't know if this is "right" but what I did was patch the init.d script, changing
build_config_string() { ...
network_get_device ifname "$interface"
to
build_config_string() { ...
network_get_device ifname "$interface" || ifname="$interface"
I filed a bug over at Issue #25315 · openwrt/packages