How can I achieve the lowest power state using hardware pins with LWB5+?

rfkill should shut off wifi transmission but you will need to disable Bluetooth as well.  

bluetoothctl power off  

 
Alternatively, you can blacklist the brcmfmac driver to not load wifi driver and power cycle board.  

Or if you have access to BT_REG_ON/WL_REG_ON, drive them low and radio will be held in reset. Then, there should be no transmission from radios for certain.