I found the culprit: WDR enabled. After clicking that off, the images are now fairly well lit (raining now, thunderstorms all day):
Hikvison 2cd-3345-i 4mm (IR only with a little street light) WDR off, Smart IR off, Iris 1/100 Gain 90, DNR 72
Hikvison 2cd-3345-i 2.9mm (IR only) WDR off, Smart...