Change main desktop to external screen after sleep mode
Solution 1:
You can change your default display to the external monitor by setting a boot argument. Note that this will disable the internal display of your machine. It is described as the opposite of clamshell mode, but this isn't quite accurate, since this works when the clam is closed as well:
sudo nvram boot-args="iog=0x0"
To clear the boot argument, either reset the NVRAM or execute the following. The former will clear all NVRAM contents, the latter will only clear the boot arguments variable:
sudo nvram -d boot-args