Re: Just speculation but ...
Common misconception. "Space" is indeed huge, but orbits are not. A satellite is like a train, speeding along a well-defined track, allowing you to know days in advance where it will be at any time.
I guess that could depend on how accurate your tracking and space weather monitoring systems are. So compensating for any increased/decreased drag due to atmospheric expansion/contraction. And to extend the train analogy, you'd still have to do something the equivalent of glueing a dart board to the side of a high speed train, throwing a dart at it and scoring a bullseye.
...at best it can make some slow and clumsy efforts to (very slightly) change altitude.
Where's your imagination? Vehicles are already fitted with anti-missile systems, so same could be extended to satellites. ERA might get interesting given Newton's 3rd law, as could jettisoning anti-missile shields. Or rely on 3rd Law effects to create an emergency dodge manouver via ERA tiles. Challenge would be either on-board systems to detect an incoming threat, or ground-based detection.
but if he wanted to kill them all, he would only need one single ASAT rocket:
I still don't think ASATs are the risk. All you'd need to do is create enough collisions to generate that cascade. More satellites, the easier it becomes to do that either accidentally or deliberately.
Yikes! Two objects meeting at 2x 6800 mph/11000 km/h would only leave a big cloud of plasma I guess. Don't be fooled by movies into thinking...
... plasma 'bolts' have much in the way of kinetic energy, recoil or all the other effects generally seen in SF. Trying throwing a candle flame (just the flame) at someone to see what I mean. Especially given the Sun already farts collosal amounts of plasma in our general direction, with about as much kinetic effect as the average Storm Trooper.
This means orbital collisions aren't just fender benders, a simple hypervelocity spec of paint can crash through armored glass, as seen on the Shuttles (and maybe the ISS too)... A stray bolt would crash through anything, like some anti-armor kinetic energy projectile.
Two of my favorite examples. NASA discovering a chipped windscreen on a shuttle, then discovering it was probably due to collision with a frozen fragment of urine. And an M1A1 tank during one of the Gulf Wars that had a very neat hole drilled right through it, probably by a new type of RPG. Again part of the joys of physics. A typical kinetic projectile would probably be a long-rod DU penetrator, but that generally relies on the properties of DU, ie fragmentation and being rather pyrophoric. Otherwise you could end up with the M1A1 example, and just leave a neat hole.
So depending on desired outcome, you want 'enough' energy to achieve the desired effect. Too much, and you'd have your plasma, which would cool and condense into teeny droplets, or do nothing unless it made a hole in something vital. Accidental or deliberate collisions would probably be the most certain way to create shrapnel clouds though.