Is this why I have to wait upwards of 5 minutes to be able to actually use my SN30 Pro+ even after it's connected? I've only ever tried this docked, will try undocked today.

In my case, I just assumed it's Steam taking it's time to realize the external container takes precedence. After three few minutes of waiting it works just fine, and the UI instantly switches between XBox/Nintendo layouts and the Deck controller's layout when I press buttons on either device.

I have also done this while in desktop mode, and while the system starts registering the controller events as soon as it connects, Big Picture still takes its sweet time, and by extension using the controller to control the desktop does too.

Have you tried updating the controller's firmware?

I wonder if the dock is perhaps emitting interference?

Aside from maybe reorientating the dock, maybe try updating the SN30 firmware?

I've also had this issue with both of my SN30 Pro+ controllers but haven't really found a reliable fix. Sometimes it works if I connect it undocked, and then plug the Deck into the dock, but that doesn't always work. It's a shame because it used to work great!