No, US directly attacking would escalate it into WW3 and it doesn't matter if Russia holds air forces in reserve or not, Russia cannot hold by it's own power against a full attack from EU and US land armies.Is it not reasonable to speculate the air force is being held in reserve to hedge against possibility of direct NATO/US attack?
Honestly the current situation is similar to the one in Kherson, only with higher stakes for both sides and better odds for AFU. Ukraine will try to establish solid borders for the salient and cut off smaller Russian troop concentrations. Russia will try to move huge force concentrations to cut off the entire salient.
The air force would be used to support this move.