Do you have an attic fan? Perhaps that would help. Here is a link explaining some of what I was talking about and this example actually shows the air handler in the attic.
https://georgebrazilhvac.com/blog/what-temperature-should-my-central-air-conditioner-be-putting-out
Keep in mind the 20 degree Delta T is just rule of thumb. That said, it seems you have a problem. 68 degree supply air is not going to cool the house sufficiently (upstairs).
At least that is my experience.