In addition to haze, at f/18 used for the first photo you are well into diffraction territory with the APS-C Nikon D5600. With a pixel pitch of 3.92 microns, the Diffraction Limited Aperture of the Nikon D5600 starts at around f/6.3. Even f/8 and f/10 used in the second and third photos, respectively, are impacted to a lesser degree by diffraction.
Diffraction has the effect of softening image detail due to the interaction of light waves with the edge of the aperture blades as the light passes through the lens. The narrower the aperture, the higher percentage of the light passing through the lens is affected by diffraction.
At 1/100 second the first image may or may not also be suffering a bit from camera shake as well as diffraction. Regardless of how good your shooting technique is when hand holding your camera, a good tripod will always be more stable.
Another factor that may be in play is lense flare, which can reduce contrast even when specular flare (those aperture shaped light spots) is not noticeable. Veiling flare reduces the overall contrast of an image. The fact that you are getting less contrast when the sun is to your side than when it is behind you suggests flare may be playing a role. The best way to reduce flare is to use a well made lens hood or otherwise block strong sources of of-axis light from shining on the front of the lens. Cheap knock-off lens hoods can sometimes make things worse if the inside surface is shiny and reflective.