As shown in the photo my MacPro3,1 is running High Sierra and it runs very well I must say.
This is the Link I used to install High Sierra, installation was a piece-O-cake.
http://dosdude1.com/highsierra/
Edit: just did the latest update from the App Store. High Sierra 10.13.1
There's always the "dark side" which seems more then willing to support your needs both with top quality highly customizable hardware and a first rate operating system, if you can believe the masses..
This is a stretch, but anything is worth a try.
Under the System Preference menu->View->Customize,
temporarily hide most of the lower pref panes and see if that might shrink the Preference window.
If it does then re-position the window and unhide the panes you hid previously.
edit:
something...
I use Free a little app called "Spark" that lets me program the Function keys and any other key combo I choose,
This app will re-program any key or combo of keys to open the optical drive.
https://web.archive.org/web/20160306215420/http://shadowlab.org/