Interesting note for future me, and anyone else who is interested. I had been trying to install Windows 10 on an old laptop with a buggered battery and it just kept failing to install. Any other operating system installed just fine, which had me questioning my install usb and redoing it countless times in different ways. I replaced the hard drive, the ram and switched power supplies (it was rebooting more than it should).
Dagnabbit, simply removing the battery was all that was required. Another day, another learning experience. If anyone else happens to know why this oddness only applies to Windows I’d be interested to know! Hope this little story of stupidity saves someone else a bit of time. 🙂