System Preferences continues to open on my broken laptop screen even though everything else works successfully on my external monitor?
Even though the monitor is broken, macOS doesn't know this and it's still being detected as the main display which is where System Preferences opens to.

One easy solution is closed clamshell mode. 


https://support.apple.com/en-us/HT201834


When the lid is detected as shut, you get one screen. A magnet placed by the top case can trigger the closing if the damage prevents you from actually closing things. 


https://support.apple.com/en-us/HT203315


A strong magnet used to be bad for floppy storage and hard drives that didn’t have shielding but now everything Apple makes basically has small strong  magnets and any fridge type magnet won’t harm things other than sleep mode. You need an external keyboard and pointer for this to work long term, though.