By any chance, could you attach the working driver you're using for the Pixel C...that works with FastBoot?
Everything works perfect with ADB, but for FastBoot Windows 10 doesn't recognize the driver installed.
I've been scavenger hunting for it
It was on xda that I found the original instructions, but the driver for this device is goofy, and you will have to reinstall it everytime you try to boot the device from fastboot, (at least from my experience). You can try this page here and I although these instructions are outdated, I would suggest these steps are valid: 2, 3, 6, 7, 8, 11, 12.
Trouble connecting Google Pixel C to Computer? Download and Install Official Google Pixel C USB Driver for Windows 7, 10, 8, 8.1 or XP PC. Also, download Google Pixel C ADB Driver & Fastboot Driver which helps in installing the Firmware, ROM's and other files.
You will manually have to look up the generic android driver in the scroll down list. Make sure you have the drivers from the latest android sdk, not any generic drivers.
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Uninstall any other generic drivers you may have installed, or phone drivers from other OEM. My phone is a one plus, and I had quite the learning curve to fix their driver incompatibility issue with old drivers carried over from old versions. Now I only use the SDK driver from google. Strangely enough, I can flash magisk via fastboot but my pixel never shows up via usb to do file transfers, etc. It did when I originally got it but it has been many rom versions that my usb transfer has not worked. FYI, you have to be in fastboot to enable the driver and not in the rom. Every time you reboot the device, it changes the driver state and you will have to reinstall it. Perhaps there is a better way, but I don't flash this device often enough to have figured it out.