Let me start by saying I don't have a greenhouse and everything I know is from research and anecdote..
I feel your pain, because I'm the same way. I don't think in our location (I'm in Yonkers NY) you can avoid frost with a stand alone greenhouse that won't cost twice what building an addition to the house will cost..
Getting over on winter is not going to happen with something that comes out of a box, but you can extend your growing season so your tropicals don't have to come in on Sept 30 and stay there till June.
You could get through more fall, and go back out around the equinox, but Dec, Jan, Feb are non negotiable with a plug and play greenhouse made of poly sheeting.
I have a 6 sided room in my house with 11 windows, and when I close it off from the rest of the house it gets into the 40s in winter, and I have 5 layers of wall. I have exterior cedar shake nailed to plywood nailed to stud, on on the interior I have plaster and lath, covered with 1/2" wood paneling That I covered with 1/2" Sheetrock and every exterior wall has baseboard heat. At the widest points the room is 18ft x 8ft and I have 30 ft of baseboard heater. I'm barely staying about freezing, and I'm not sure it matters much, because even if they survive the cold, they're basically knocked out, dormant till it's warm.
I think a greenhouse is an excellent idea, but only to extend your growing season, store dormant roots, tubers and corms you will repot, and it can allow you to take cuttings in the fall that can come in small, and go out early so they don't overtake your life inside
