How do I Propery Configure Display Scaling on macOS (with a 1440p External Display) to Reduce Eye Strain?
The solution requires SwitchResX and to follow this guide - How to Mimic a 2K Monitor as “Retina Display” in macOS Sierra Using HiDPI. The high level steps:

Install SwitchResX.
Disable SIP.
Create a custom 4k resolution (3840 x 2160) in SwitchResX using the Scaled resolution option.
1920 x 1080 HiDPi becomes available in Current Resolutions of SwitchResX.
Enable that resolution.

The image is now going to my monitor at 2560x1440 with the HUD scale of 1920x1080.
(Thanks to @Tetsujin &amp; @sfxedit I've been able to get the monitor displaying how I wanted.)