How can I create a bootable windows 10 usb installer on Mojave?
The easiest (and best way) is use mediacreationtool.exe, but that requires having access to a computer running windows.

You can use dd to make a bootable Windows 10 installer disk (I've done it, and it worked), so just make one using an .iso from https://www.microsoft.com/en-ca/software-download/windows10

As for Boot Camp Assistant, it creates a drive with an apple bootcamp tool, so you'll still want to create a drive after successfully installing Windows.

There is another option for installing Windows, which is this except instead of an external disk use the desired partition (and if it is another computer use firewire targetdisk mode I guess?)