As for IEDs, the drones used in the Ukraine war use what look to be very effective munitions, and they seem to be only effective against a single target really.
Is that really of concern?
And its not like drones that can carry things are cheap, nor is the way you are hypothesizing about them being used.
Easy? Is sarin gas freely available at big-box retailers?
All of the weaponized forms involve some pretty nasty flourinated precursor chemicals which themselves are quite toxic -- and controlled.
e.g. isopropyl alcohol.
I doubt the precursors to nerve gases(the worst ones, anyway) are readily available, but they are probably a handful of undergrad-level reactions away from easily available chemicals.
It's a bit further than that, and all of the intermediate chemistry is toxic.