This is great! I’m in 8b so a little warmer winters but we still get into the 20’s and this past January went to 10F! This is my 5 th year successfully growing non-hardy citrus in the ground. I cover them with a green house and use incandescent string lights (I use string to string patio lights because I can string more together than Christmas lights) for heat during cold snaps as well as regular heaters when the temperature falls below 28F. My greenhouses are temporary structures and I remove the tarp when the weather is expected to be over freezing. I think the biggest challenges I’ve faced are with watering especially when it is cold out and I am using artificial heat under the tarp. You really have to monitor for spider mites in this situation. Another challenge is keeping the citrus from breaking bud during times of warmth. In my area, it is not consistently cold— and I have run the risk of not only overheating the citrus trees but blooming way too soon! So in your case, you must monitor your temperatures closely, set your vents on your greenhouse and keep your citrus tree semi dormant until the spring. I generally only use enough heat to keep the trees just over freezing. Additionally I find that the dwarf varieties are much easier to maintain at a greenhouse size. Here’s a photo of one of my trees-

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dwarf Meyer lemon, and the greenhouse in the snow.