I had issues with getting my Windows computer to recognize the phone in fastboot mode, too. It took me a while until I found out that sometimes you really have to point your computer towards the driver that is already installed but not connected with the phone. I think this explanation covers what I had done at that time:
https://ifindhub.com/install-fix-adb-fastboot-drivers-windows.html
Step 6 and the following are the ones that made the difference for me.
It is explained similarly in this thread here in this forum:
Forcing the driver seems to be important.