I can confirm that these USB-C leads work well hardwired to a 12v power source. That’s how I’m powering my Rock 5b (running Armbian).
You can also get the voltage currently connected with this bit of magic.
printf "%.1f V\n" $(bc -l <<< "$(</sys/devices/iio_sysfs_trigger/subsystem/devices/iio\:device0/in_voltage6_raw) / 172.45")
The 172.45 part is from ROCK 5B Debug Party Invitation