Updating SiLabs UZB-7 SLUSB001A USB Stick (EFR32ZG14) US.
This got me 90% there: https://github.com/zwave-js/node-zwave-js/discussions/2544#discussioncomment-1323267
However you actually need to install the "Gecko SDK - 32bit and Wireless MCUs" (which at this time was 4.0.2)
Firmware as of writing: C:\Users\<your windows user here>\SimplicityStudio\SDKs\gecko_sdk\protocol\z-wave\Apps\bin\gbl\ZW_SerialAPI_Controller_7_17_2_406_EFR32ZG14_REGION_US.gbl
Is then able to be written with the PC Controller.