[hayden55]

Oh okay so the basement is above ground. Well seal the floor or any surface connected to ground with epoxy waterproofer to start. Also, you want to unblock the upstairs return as it will return all of the hot air sitting up there, and block the return air in the basement, but your humidity levels down there don't allow that. What you want to do is adjust the main air damper to the upstairs and block it to divert more of the air to down stairs. It is more effective.
But yeah a good manual J calculation gives us heating and cooling loads required for the house and on a per room basis. Not using one is like me design shirts based on height and i get two 6'2 people but one is fat a old guy so he ends up in a muscle shirt flexxing man boobs and the other is a 6'2 skinny kid so he ends up in a short dress. lol
The new hvac units are called minisplits and they are air source heat pumps. Super efficient, but e is more expensive than natural gas so on the year it comes out that they are roughly half the cost to run over our stuff from what i looked at. But they can also run at variable load and speed so they can run all day which is what you want. My AC/Heat is double oversized for my house so it hardly runs at all which is also very bad especially for humidity removal in the summer lol. Also, the new ones can run down to -40 on some of the new models so you just have to calculate which one will work for you. Coefficient of performance is also variable with temp so you want one that will work in winter on top of all things with these types.