Cuz as Friend Or Foe makes a point of showing, Plankton and Krabs' rivalry is a personal one that stems off of their former friendship. I can imagine Plankton being one who's very much driven when it comes to his schemes and would love to be the one who takes down his long-time rival/arch-enemy. That means, if Plankton himself does it, it's a personal victory, and thus has more meaning.
For Plankton to hire someone to just retrieve a patty for him to analyze, then that'd just be an indirect victory that lacks all the personal beef Plankton has with Krabs, so where would the fun be in that for him? Yes, I believe Plankton would rather have Krabs' ruin rest in his own two stubby hands as a means of personal payback for all the years he's spent being the inferior restaurant owner.
SpongeBob and Squidward can also be considered Plankton's enemies in this aspect too; since they work for Krabs, for the protozoan to attack his enemy's workers, Plankton would be dealing some damage in Krabs' forte. So...i.e. whenever Plankton uses SpongeBob's naivety, him being a nemesis helps add to the satisfaction of a possible victory.
And as for the whole Karen/Karen 2.0 thing...maybe the original Karen's hardware just wasn't compatible with the technology Plankton implemented in 2.0.
...or something like all that. It's a cartoon. Plankton's plans make for comedic fodder a lot of the time. :P
