Unfortunately, it doesn't boot to either recovery nor rescue, each time stating there is no bootable OS available.Are you clicking on Flash boot or Flash Pixel Phone?
If you've been clicking on Flash boot, then click on Flash Pixel Phone
Otherwise try this:
While in fastboot mode type
the phone should reboot and you should see an Android with it's cover openCode:fastboot reboot recovery
I did sideload from Recovery previously (prior brick) updates and all worked.
If the sideload you had in mind was Recovery based booted of the phone from either bootloader or "fastboot reboot recovery" which has same effect - then it doesn't work. Really appreciate you trying to help.