You can set up Windows 10 or 11 onto an external USB drive (I heartily recommend a good USB 3 SSD) using either Hasleo's WinToUSB (via another Windows machine) or Twocanoes' Winclone on a Mac.
Download and burn the Windows ISO to your drive via either of the two programs.
When finished, insert the drive into your Mac, hold down the Option Key and select the Windows drive.
Boot into Windows, configure as desired, run updates, etc, etc, etc.
You need to separately download the Windows Support folder (which you can do via the Bootcamp app) and install it once your Windows is up and running.
I have been doing this for several years, as my iMac has a Fusion drive, and when Bootcamp partitions it, it only partitions the HDD part of the drive. Consequently Windows is running from a HDD and is very slow. Installing to an external SSD is much faster.
BTW, burning a Windows ISO to a SSD via WinToUSB will give you a drive that will boot up either a Mac or PC.