External Boot Camp SSD not showing up in Startup Manager or Startup Disk, and cannot access from recovery mode

I'm installing Windows on an external drive (Samsung T5) since my computer doesn't have terribly much room for a full Windows install with my productivity and gaming suite. I followed the instructions here, and it did all the right things, except for right now - it doesn't display the startup disk in Startup Disk under System Preferences, or in Startup Manager with the alt + Restart. It was visible in Recovery Mode, however it said "The bless tool was unable to set the current disk". I've changed the security settings in Startup Security Utility to the "all signed utilities" (medium setting) and also allowing external disks, so it should work - but it's just not showing up. What are some steps I can take to troubleshoot this? (I've checked the drive is formatted as NTFS after the Windows install, and all the program files are there - the computer's just not recognising it.)

All details are below - more than happy to provide more information. Thank you in advance for your help.

System info:

macOS Catalina 10.15.5

MacBook Pro 16"

Windows 10 64-bit Education Edition