Re: uneven heating and cooling
The good news is that you don;t have issues with heat rising in winter is seems, so the house must be reasonably air tight. The bad news is that you need more airflow upstairs. You'll need to add balancing dampers to the start of the branches if you have access to the ductwork.
Otherwise, adding a return upstairs will help a little, but you might need to install better quality supply registers. They aren't cheap, but a higher quality register will have a lot less restriction and will flow more air.
The second issue is that like the system in most homes, it's likely oversized. Short cycling will make temperature difference worse. You're better off undersized than oversized in most cases. Because a undersized system will simply fall behind a couple degrees for just a few hours a year. IT'st suprising how little heat you actually need.
I have a 3200sqft 1925 wood frame home in SE Iowa. Attic is well insulated and I have good storm windows and it's fairly tight in terms of air leaks. I only need a total of 75k BTU's to heat my home when it's -2F outside. A smaller 2400sqft early 1900 home with solid stone walls for example might only need a 45k BTU furnace with it's thermal mass. It effectively raises the design temp.
So, if you ready for a new system, or when you are, get it sized correctly using a proper load calculation. For now, change out the registers and make sure they are all open and in the warmest room downstairs, add a damper at the start of the branch or partly close the register.
1925 Two-Story Stucco Beaux Arts Neoclassical