Hi Josh, this solution did not work for me. Per your post, I changed the Graphics Setting to High Performance and checked to box to run as administrator, but when I try to share the entire screen via Teams remotely, it still looks black.
I'm using RealVNC to remote in to my PC, and the privacy settings are such that the screen is blanked when I log on. With this setting activated, I cannot share my entire screen (looks black), but I can share individual applications.
If the setting is NOT activated (screen is not blanked), it works fine and I can share my entire screen, without changing the Graphics settings or running as administrator. The caveat is that people in the physical location of my PC can also look at my screen and see what I'm doing, which is not desirable.
Here's the interesting bit -- If the setting is not activated, I turn off the monitor, and then share, it still looks black. If I share and then turn off the monitor, the shared screen is still displayed, but it is static and does not refresh.
So it seems that the monitor has to be physically turned on in order to share the entire screen. The best workaround I've come up with so far is to share the screen of the client PC, the computer I'm remoting in from, and in this case, Teams would look at the VNC application window. Another option might be a pseudo display connector, to try to trick the computer into thinking the monitor is turned on when it's actually not. I have not tried this yet though.