Microsoft is being a dick. Booting from an external USB-attached SSD works fine once you work around the installer limitations.

When I did this I ended up basically partitioning the disk manually with diskpart and installing Windows manually using the command prompt. I used this blog post to guide me:

web.archive.org/…/install-windows-10-on-usb-exter…

But I remember I had some issue that made me start over… what the heck was it. Something like the modern Windows 10 image being larger than what the author had encountered. Ah - I think the issue was the WIM file was over 4GB and wouldn’t fit on the authors 4096 MB “Recovery Image” partition. So make that one larger (check the iso for the wim file’s size) if you plan to create a recovery partition.

Another bookmark I made at the time was this, I think it was mainly for the command listing the SKUs supported by the WIM file, but no guarantees, it’s been a while: gist.github.com/…/e8ce6306a038902df6e7a6d667544ac…

Good luck if you decide to try!