As far as AG South is concerned, Halder was smoking crack. With only one Panzergruppe [Von Kleist’s], Rundstedt lacked Bock’s ability for encirclements on a large scale. Additionally, he was constrained by geography to a much greater degree than Bock. The Dnieper ran generally east to west on his left flank for a good distance. To the south was the Black Sea and Sea of Azov.Hence, no great turning movements.
He had to deal with a land bridge on his southern flank in Russian hands,the Crimea, which was reinforcible by the Russians until taken by the Germans. That tied up one of his armies [the 17th]. So basically, aside from his Romanian contingents, his striking power was basically Kleist and Reichenau. But Halder is still focused on Moscow.
If it was up to Halder, Moscow would have be the only objective. You’re right though. He really is missing just how precarious his position is with AGS and really is underestimating the size of the Russian reserve for that matter.