Final edit: this worked, once I used the correct directory. Note that Steam gives no indication that it found the files except to skip right to “Verifying”.

That folder only contains the folder “Steam Controller Configs” on both drives.

Edit: found them at ~/.local/share/Steam/steamapps/common on the old drive

Hmm this might work, except I have no space left on the 500GB drive.

I saw a couple mentions of that, but never found the setting to add it…

Oh no! We suck again!