It was suggested to me that the requirement for X11 was likely part of the problem so I tried removing
(requirement '(x11-display)) from
redshift-shepherd-service but that resulted in an error from string-append (trying to append #f).
So I commented out the adjustment of the DISPLAY variable in #:environment-variables and that resulted in, finally, no errors but
Redshift, nevertheless, failing.
Since the environment variable was returning #f, I hardcoded it to "0:0" (I haven't tested if this works without the DISPLAY variable) and set WAYLAND_DISPLAY to "wayland-1" and that seems to've done the trick.
I'm not sure how this would be adjusted but it seems a Wayland option need to added to the service so non-X environments can, also, utilize the service.