The fact that your non-Retina display was much more tolerable on dark mode suggests it may have been a display calibration issue. High Sierra did look great on my 4k 27" Dell, but only after doing a custom calibration using my xRite i1 Display Pro spectrophotometer. Indeed, I've found all non-Apple displays need to be calibrated to look good on any version of MacOS, and that Apple displays benefit from this as well (though the difference with them is not as striking).IMO it was Yosemite through High Sierra that were the worst looking MacOS versions on a non-Retina display. When the full system-wide dark mode came about that actually got it looking better, at least to my eyes.
Also, High Sierra did require ≈160 ppi (=4k @ 27") to look good. The last OS on which a low-dpi (~100 ppi) display looked good was Snow Leopard. They changed something after SL to require a higher resolution (forcing me to upgrade to the 4k 27"), and then changed things again after HS to require a higher resolution still (forcing me to upgrade to a 27 5k).
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