a tight place, scowl, approaching
蹙 centers on being pressed into a tight, uncomfortable space, which can manifest physically as a scowl or a sense of something closing in.
蹙 combines 戚 (a type of axe or relative, here likely contributing the sound) with 足 (foot), suggesting a foot being pressed or cramped, leading to the idea of tightness and frowning.
Imagine your foot (足) being squeezed by a relative (戚) in a tight space, making you scowl as they approach.
For シュク, picture being shushed (シュク) by someone as you scowl in a tight, quiet place where you can't move.