The 42 campaign start is flawed I think. The Soviets learnt from their mistakes in 1941 and mostly legged it in June/July 42. I think there was one decent sized pocket near Stary Oskol. Also army group A didn't start their offensive until July 9th. The campaign needs adjusting so that some of the Soviet Southern forces can withdraw before the Axis can get behind them.
As for strategy, my first instinct would be to strategically surrender

The thrust towards Voronezh is dangerous because it unhinges a possible Don defence and also threatens a move towards Tula/Ryazan. So like you haven't enough to worry about in the South you also have to worry about Moscow and the Western front, which is unhelpfully mostly static.
My strategy for the South would be to concede plenty of territory to get away from Axis supply points to give time to rebuild a strong army. Something like tambov-Bugachar-Chir river-Stalingrad. I would delay where possible with pickets, but not risk my main force.