How to install Windows without Boot Camp Assistant for iMac

Hi. I have ben trying to install win10. I have macOS running on an external SSD ( Ok, I have a 13,2 iMac that has two internal SSDs funning as a Fusion drive, BCA point blank refuses to attempt to create a partition on that ). I have had Monterey, Ventura and now Big Sur installed on the external drive.No matter how many time I try, BCA tells me the drive cannot be partitioned, and yet there is a partition called Bootcamp now present on the drive. BCA will go no further, and will only remove the partition if I click quit. OK, I've had enough. I have all the helper files etc. Can I install win manually? I only need win for one thing. At this point I reckon it would be easier to. just buy a cheap rubbish pc laptop, but I don't want to.

Any ideas?

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