The springs you have are terrible riding springs as confirmed by others . KYBS are the worst and most harsh riding shocks I ever put in a mustang . I had tockico blues with
eibach pro kit springs and it rode like a slightly lowered car but very close to stock . I’ve also ridden in cars with the h&r sports and they were awesome as well . The one solid bushing on one side of the arm doesn’t sound like where your issue lies . Are solid joints clanky ? Sure . Would they jar you a bit on a huge bump ?absolutely . They are not going to make the whole car ride harsh though . I run totally solid bushings on my upper arms and rear bushings and my car isn’t that harsh . Slightly clanky but not harsh
IMO your harshness sounds to me like it’s that spring and shock combo.