I want to turn off my displays when I lock the laptop.

Display 1 is USB-C
Display 2 (primary) is HDMI

Research said to use task scheduler to run "C:\windows\nircmd.exe monitor off"
I installed the program and set up the task.

Trigger: When workstation locks
I'm currently running the task in Windows 10 mode (I tried the default - Windows Server 2008 and Vista.  That also didn't work.)

When I press WIN+L the displays go blank, then turn on, repeating this several times until finally staying on.

I also tried running this batch file.  Same thing happens.

RunDll32.exe user32.dll,LockWorkStation
C:\Windows\nircmd.exe monitor off

Am I fighting a power setting?
Missing something in the command?  (The instructions did not say to use a / in front of the 'monitor off')
Fighting something in Windows display settings?

I'm at a loss.  I have no problem abandoning the nircmd.exe program if you have a better way.
I'm fine with editing the registry if that's what it takes.

How about setting them to go off after a certain amount of inactivity? Like 10, 15 or 20 mins?

I'm almost certain you can do that in the Power settings. Mine shut off after an hour.

I tried that.  I set the display to turn off after 45 minutes when on AC power.

The USB-C display shuts off when I simply lock the computer, but the HDMI display never does.

Do your external monitors have their own internal settings for dormancy?

I checked.  They do not.