I have a two story home with one five ton unit. Of course, the upstairs area is hotter than the downstairs, but in the summer months, with my unit running all day, it gets up to the mid 80's upstairs, while being 75ish downstairs.

I'm trying to find a way to help out upstairs, but I don't know which route to go. The air intake is at the top of the stairs, which is just about in the middle of the upstairs floor plan. There are two bedrooms and a bathroom upstairs, and each has its own vent. I feel that part of the problem is the air coming from the vents is very close to the intake, their distances being about 10', 12', and 15' away.

The other thing I was contemplating was insulation instead of worrying about system changes. There's only insulation on the interior walls, none such as spray foam on the roof though.

I was looking through adding a ductless system upstairs to help, but with the intake being in the center, do you think it would actually help both bedrooms, assuming the BTUs are inline with my upstairs sqft?

Many older two story homes have similar issues. In the summer you should close all first floor supply ducts and open all upstairs supply registers. You also want to cover about half of your first floor return registers and leave the up stairs returns alone. Your ductless system would help but try what I talked about first, heck of a lot cheaper