You absolutely need a thermometer. It doesn't have to be fancy of expensive; just get one you can hang on a wall or set on a counter. Something like this would do you - we have several of a similar type.
https://www.hubert.ca/product/...
Our greenhouse is heated all winter, both with hot water in the floor and air driven by a fan from the house, so it keeps a pretty steady 65+ degrees F, warm enough for the hydroponic greens.
For tender seedlings, we also built a special tent. I start the seeds in the house (on a bookshelf near the stove) and move then out to the greenhouse when they have their second pair of leaves and look big enough.
The tent has grow-lights that I need to go turn off now - 12-hour day. Underneath, we have heating pads, but they haven't turned on at all this year. They're controlled with a wee heat sensor that goes into the potting soil.
I'll take some pictures tomorrow.
