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?)

Unetbootbin should work fine for Windows ISO less than 4G with FAT32 partition. However, Windows 10 ISO with latest update is larger than 4G so you have to format the USB to exFAT32 before burning.

Here are the tools that work fine on Mojave 10.14 5 on my MacBook Pro:

https://www.balena.io/etcher/

https://www.uubyte.com/iso-editor.html

You can also set up a Windows virtual machine on Mojave and burn the iso to USB with Rufus or Windows Media Creation Tool