Kursk was not irrelevant; for the Germans, it wasn’t about “winning” but rather getting an operational stalemate in the East. A draw if you will.
The main problem for the Germans wasn’t Kiev or Smolensk. The die had already been cast by then. The real problem was in the brain of Chief of the General Staff Franz Halder, who cast the die. He sold Hitler and his generals on the idea of defeating the USSR in one campaign season. A look at the map should have told them it was logistically impossible to project their power that distance in such a short time. Instead, they should have planned for at least a two-year war.
Meanwhile, the Red Army had months to prepare defenses in depth against the anticipated attack.