Unless you have already started the plants you intend to grow in this future greenhouse, I would say you're likely too late already for a winter harvest and should start thinking about spring. Winter crops have to get most of their growing done before the end of October, after which the days are too short without supplemental heat and light for plants to put on a lot of growth. So if you start plants in October, they will grow a bit, then sit there and wait for the Persephone months to be done, and when light gets above about 9 hours a day again, they'll resume growth. Spinach is commonly overwintered for an early spring harvest.
Consider row cover inside the greenhouse for a layer of extra protection.
Grow crops that can take some frost and you have less to worry about. You just harvest them when they are not frozen.
Thermal mass is your friend.
Water, rock, earth, brick, etc.
A properly prepared
compost heap inside the greenhouse will produce a lot of heat.
Partially sunken into the ground is good.
Direct overhead light on the plants is key.
Worry more about cooling than heating! You will understand this once you've worked in a greenhouse for a while. Top vents, and ridge vents are great, with low vents for fresh air input.
Anyone's first greenhouse design will be full of mistakes. that's okay, it's how most people learn.
Passive solar residential architects don't know a thing about designing a greenhouse. It is specialized knowledge.
Check out Eliott Coleman's work, such as Four Season Harvest.