How to create "Eternal Night Shift" on macOS Mojave?

I've already set Night Shift to end at 9.00am and to start at 9.01am

Then you should change the time to when you're not awake. For example 5:00 AM.

You can try using f.lux and setting the Daytime, Sunset, and Bedtime color temperatures all to the same value:

Using f.lux also has the added benefit of allowing you more control over color temperature changes than Night Shift.

You could just try a manual tweak in Display Calibration.

System Prefs > Displays > Colour > Calibrate.

You could go right through this & set everything to your satisfaction, but for just a quick test, skip through to the second page - Select a Target White point.

Uncheck Use Native white point & drop the slider from D65, which it will probably be at default, back towards D50. As you slide left, your screen will 'warm', red/yellow light to the left, blue to the right.

If you like this method, but it doesn't feel quite right, you can do a lot more balancing work if before you start, you hold Opt as you click the Calibrate… button. This will open up more options [which you'll see in the left side bar] as you progress through the Assistant.