I have Calvin Coolidge and Andrew Jackson added on my short list.
I personally believe William Henry Harrison may have been offed by early deep staters not so much because he of his banking agenda, but because he wanted to reverse the spoil system of executive patronage.
Find it hard to believe someone used to fighting in the wilderness succumbed to a rain/cold from just their inaugural activities - no coat & oldest president till Reagan or not.
I agree with you that William Henry Harrison was probably assassinated, bringing the total to 5.
Together with the 6 known assassination attempts when Presidents were actually targeted (and in most cases shot at or shot) by assailants, it is clear that President of the United States is a dangerous profession.
11% assassinated in office
24% targeted by armed assassins, successfully or unsuccessfully
Fortunately, most of us do not face such lethal odds as a result of our jobs.