I know you posted in greenhouses, but as the other reply said, much depends on what you want to accomplish. You have a shorter but not insignificant growing system now.
For extending growing seasons, starting earlier and going longer, temporary tunnels can do that without filling the area with greenhouse.
For just a winter plant refuge and place to start seed for the spring, a small greenhouse is sufficient.
Of course, for growing that requires a controlled environment, a full size greenhouse is appropriate. I need one here for year-round hydroponic growing through weeks of 100+ temperatures and still some freezes and short winter days.
Two hours a day of true full sun is a challenge with some crops. But there's a lot of slack in most light requirements. Tomatoes, for instance, may merely suffer some loss of production in part sun. One thing a greenhouse can do where full sun hours are limited is provide a place where artificial lighting can be brought to bear.
Grow lights are not just for hydroponics. They can be used as adjuncts to natural light in a greenhouse, with the bonus of providing sun equivalent in bleak winters. I have LED strips throughout the greenhouse supporting plants on days of weak sunlight. They are mounted to high and are too few to be the sole light source for growing, but they provide a real boost. They do, though, require a timer to give plants the necessary dark period each night, without which they have limited ability to build with the energies built up during the day.
Grow lights were once sort of expensive, but the increased use of hydroponics for home growing (and not just dope, although that's where a lot of the business is) has brought the Chinese into the grow light business and made effective artificial light growing fairly cheap, and LED lighting is cheap to run.
It's worth looking into, because it can mean home grown lettuce and tomatoes all year. Even things like melons. I'll never again grow lettuce in the ground. A "rain tower" occupies one square foot and can pretty much keep me in salad without sharing with the bugs and such. And a second can keep me in strawberries. (Google RAIN TOWER.)
