"Click restore to remove the Windows partition and restore the space back to MacOS" I made a Windows partition on Bigsur install to make Windows quicker and easier to install later. However, when I run bootcamp the only option I'm given is to get rid of that partition and get it back to OSX. I don't trust that since last time I tried something like this I couldn't even boot and lost everything.
Is there a safer way? For example, can't I burn Windows to a USB drive and then boot directly from that to reduce risk.... then install to the spare partition.
The only problem is that UUByte doesn't seem to work on Bigsur (greyed out drive selection) and Etcher can't get that Windows ISO into a bootable state.
Meanwhile, Parallels would need messing with ktexts and I don't like the potential instability of that.
What's the safest way to proceed? I have to run a Windows program to downgrade my phone via USB.
The safest way usually is to use the Boot Camp Assistant to install Windows 10. However, you can use a USB flash drive if you wish. Below outlines the procedure for creating the flash drive. You may have to tweak the procedure depending on your model Mac.
The macOS operating system includes all the applications necessary to create a Windows 10 USB flash drive installer. Below are the applications and their use.

The Disk Utility application can be used to erase a 16 GB or larger USB flash drive. The name, format and scheme should be BOOTCAMP, ExFAT and Master Boot Record, respectively. See the image below.


The Boot Camp Assistant application can be used to download the Window Support Software. This download will include the Windows 10 drivers.

The Safari application can be used to download the latest 64 bit Windows 10 ISO file.

The DiskImageMounter application can be used to mount the Windows 10 iso file. This application can launched automatically by using the Finder application to open the ISO file.

The Finder application can be used to copy the contents of the mounted ISO file and the Windows Support Software to the flash drive.


Since you have already created the partition for Windows, you can EFI boot from the flash drive and install Windows 10.