I'm a bit confused by the question.

Do you want to write the installation files to a USB drive? There's dd, KDE ISO Image Writer, Balena Etcher, Gnome MultiWriter, etc.

Do you want to boot a full windows installation off a thumb drive? You would want to look at Ventoy or WinToUSB.

Would you want to run windows at the same time in a VM? Thats essentially what I do when I need to run a specialty windows application.

Windows To Go was discontinued back in 2019 so it's not really something that has been maintained or updated for a long time

https://en.wikipedia.org/wiki/Windows_To_Go#Discontinuation

Apps like Rufus (Windows only) are still able to create that sort of boot USB but it's sort of a non supported feature, wouldn't be surprised if it just stops working one day.

On Linux Ventoy is often used for this - it does have a persistence plugin but not for Windows https://www.ventoy.net/en/plugin_persistence.html

I haven't tested this idea, but maybe you can run a Windows VM within Linux, enable USB in it, download Rufus in it, then you can create your non-official Windows To Go boot disk that way? Could be something to try if you never find another solution.

Why not just run Windows in a VM?

You could just pull your drives, install a nvme and install windows. Then pull that drive and put it in an enclosure. Then choose that drive when booting. Idk if that works on modern windows but the install then pulling the drive used to work on 7-

I don't think you need to involve Linux at all if you boot the official windows installer. I would just install the SSD as the only drive internally and install to it, then put it back in its enclosure.

Someone already told you to use a vm. They’re right, but if you insist on booting to actual bare metal windows instead, consider devoting a handful of gigabytes to a dual boot setup. It’s really easy to do and will be much more reliable than the way out of service windows to go.

If you have a specific piece of software you need to run I may could help you figure out the vm setup for it. Windows VMs are my solution to running cad packages.

Ventoy supports any iso, including Windows

I don't know about windows to go. But rufus? Isn't it just writing images to usb drives? Sounds like you're looking for the dd command. You can write images with

dd if=/path/to/my/image of=/dev/[insert device here] status=progress

Just in case and I know this may sound like a duh type question and I apologize if you have already exhausted it. But you have tried installing it with wine and play on linux?

This is probably an inefficient solution, but I just did this the other day using dockur/windows as a VM and passing through the drive. It's really easy if you have docker experience.

Once the VM has installed Windows to the drive you can just boot from it as normal (whatever the VM does to Windows to make it wanna boot in a VM let's it boot off the USB)

Here's a link the docs for this: docker/windows

Let me know if you have any questions :)

I used Rufus to make Linus boot USBs for AGES

Are you planning to use the SSD across multiple computers?

Boot to WinPE and use Rufus to create one maybe? Not sure if WTG still works though.

I personally never used WTG. If you're planning to use this across multiple machines/hw configs, I don't think the normal windows can handle it as you'd have to install the correct drivers every time. I recall there being some hardware profile thing at bootup but not so sure. (that was XP)

I'm sure windows activation will complain, but you should be able to dd your windows partition (or disk) over to the external disk, set up a bootloader (windows can do this, but something like grub or syslinux I know would work to hand off to the windows bootloader)

I don't know anything about bitlocker stuff, probably needs to be decrypted before this can work.

That's what I would try, even though it's not wrapped up in a single tool.