Installing Tahoe on External SSD

Hi all. I'm trying to install Tahoe 26.5.2 on an external Sandisk SSD. I erased and formatted the SSD correctly using Disk Utility. I downloaded and launched the installer, ran through all the usual installation steps. Progress bar advanced to 100, then...

After the second restart, my Mac Mini has booted to my existing install on my internal SSD.

Yeah, I tried setting the external SSD as a startup disk, but I get the message that MacOS needs to be reinstalled on that device. Okay, fine, I can do that... I go into boot options and try to install MacOS onto the ecxternal SSD. When I do that, I get this message: "this volume cannot be used because it has a secure token without a user."

According to this Apple support forum topic, I'm not the only one with this issue.

Has anyone successfully installed Tahoe on an external SSD? Any tips or tricks would be appreciated. Thanks.