In case it matters, I am running the latest version (8.4) of Camtasia, the latest video card drivers (for a Radeon R7 360) and the latest updates for Windows 10. My system has a SSD and 16GB of RAM as well.
Have you tried recording with Logitech's software and then imported the footage into Camtasia? I'm guessing it's just a matter of Win10 being so new and either Logitech or Techsmith not playing quite nice with it.
I know that we could capture straight through Final Cut Pro, but always chose to use the video card's native software and then converted to import into FCP.
(I could easily be wrong about the Win10 updates BTW.)
If you disable the 'right-light' setting, does 30 fps become available? (from the tip in https://forums.logitech.com/t5/Webcams/Recording-at-30FPS-with-the-C920/td-p/832334 )
My support ticket (German):