The one downside is that you need multiple missions to observe just single target. The mission could potentially be modified to be good for more than one target

The one downside is that you need multiple missions to observe just single target. The mission could potentially be modified to be good for more than one target but at the cost of length of the mission. The path of drones would have two intersect two points that need to be reached to observe two targets, but that would require most likely less hyperbolic (meaning slower) escape trajectory. The major benefit is that we don't need to invent anything new -- all required technology already exists. And the mission wouldn't even be that costly (at least in comparison). Just select the target, build (a dozen?) drones, shoot them on a right orbit into the Sun's gravity well and wait something like 20 years until they reach destination.