add a secondary storage device to my steam library.

You mean have more than one steam library? That’s a steam setting. Nothing to do with KDE. Gnome, Debian or Fedora.

The flatpak works,

Oh. There’s your issue. Don’t run steam as a flatpak, there might be sandboxing issues.

EDIT: MF did you read the page you downloaded stuff from:

Note: To add a game library on another drive, first you need to grant the app access to it:

flatpak override --user --filesystem=/path/to/your/Steam/Library com.valvesoftware.Steam