Installing Windows (Bootcamp) on an external SSD (Intel Macs)
For others struggling with this ... there are many methods to be found online, but they all seem very complicated and involve third party tools (e.g. VMs) - some which you have to purchase - and I just could not get them to work.
After more research I stumbled on this method which requires no third party tools at all, but simply uses Windows command line tools (diskpart).
I used this method to install Windows 10 in a new partition on an external 256GB SSD which also had a bootable 128GB macOS partition (Mojave).
The details are given in the Answer here: https://apple.stackexchange.com/questions/384355/drivers-not-working-on-windows-10-installed-cli-process-on-external-ssd-runnin
A word of caution though: DO NOT blindly follow the steps in using diskpart. You must adapt them to the partitions that exist (if any) on your target volume.
It's a bit tedious to type all the long cmd text - but it works like a charm.
Note also that installing Windows this way does not result in a portable drive that will boot into Windows from any Intel Mac. It may boot, but Windows will not be Activated (but you can still use it just fine but without personalisation and with nags to activate).
For example, I installed Windows using an Early 2015 MacBook Pro which had Bootcamp installed and where Windows was activated with a digital licence. The licence was copied to the new SSD and Windows was automatically activated.
When I tried to run Windows from the SDD on a 2017 iMac, Windows recognised the hardware had changed, managed to reconfigure itself to suit, and booted up, but without Activation.
When I tried to run Windows from the SDD on a 2018 Mac mini, it could not adapt to the new hardware configuration and simply would not boot at all.
MacBook Pro 13″, macOS 12.6