The Windows 10 installation ISO downloaded directly from microsoft.com does not need the internet to install correctly under any circumstances. You also don't

The Windows 10 installation ISO downloaded directly from microsoft.com does not need the internet to install correctly under any circumstances. You also don't need to do anything special to get it working. However, I opted for the inclusion of a completely optional disk driver during install time to improve performance and sustainability. You can provide a QEMU virtual CDROM with drivers during installation if necessary, and do the rest of your setup with QEMU native Samba file transfers followed by software installation. I use the QEMU virtual CDROM to provide VirtIO drivers during Windows installation for super fast virtual drive I/O and DISCARD support. -drive file=windows.qcow2,index=0,media=disk,if=virtio,discard=unmap discard=unmap is absolutely essential to keep your QEMU QCOW2 file on your host from growing excessively as you delete stuff in your Windows guest. This is the main reason I use VirtIO. To learn more about QEMU I recommend the Arch Wiki page on it