Su-35/Mig-31 whatever are not comparable to modern AWACS at all, pure fantasy.
Not sure A-50a is capable of this, these drones are low flying, small RCS. Anyway, only a few A-50a are available.
They are AESA using an X-band frequency with look down mode.
AEW or at least the A50 kind, uses long frequency which does not diffentiate well against ground clutter. If a drone is flying low it would be indistinguishable from the ground due to the lack of resolution. Which is why they fly low in the first place.
Look down pulse doppler mode compares and contrasts the radar reflections of moving objects versus stationary objects. Because drones are small, high resolution high frequency is needed to differentiate that otherwise you can't tell it from a bird because to low frequency radar they are all like one pixel blob.
Furthermore the radar paths of the drones are tracked back to their original sources. That's where the counterstrike begins. As the drone launchers are using trucks, therefore they are mobile, the next thing to strike are garages and parking lots that can hold trucks, and gas stations that can fill them up. EW also determines where the source signals are, and in due course these are targeted. Also targeted are electrical substations in that area.
Noting the history of the tracked paths, you will know where to deploy your mobile ground fire units the next time. The drones fly low enough they can be taken down by 12.7mm machine guns and even by clustered AKs.
The kind of AEW you actually need is something like the Chinese did with their Y-8s which has large fixed chin AESA radars facing sideways instead of the revolving type.
In any case, the Russians don't have such Y-8s, the main issue with Su-35 is they have limited loiter capability but it's better than an A-50 for this purpose. Also someone also recently posted an Su-30SM firing R-73 missiles at the drones which means that the drones are observable and trackable by radar. The Russians do have large stocks of expirable missiles that you might as well use them. It also means the R-73 can still track the heat from the drone's engines, yes for this purpose you are using a two stroke moped size twin engine. For more note, even if the radar can pass through the styrofoam, it will still pick up the engine.