The one kind of gardening advice that's always good is "try it and see".
Since there's no wind inside a cool greenhouse, maybe you could keep your seeds warmer with tiny hoops draped with plastic film. Or a clear plastic bag pulled around the trays that have not yet germinated - like a micro-poly-tunnel INSIDE your greenhouse. That would eliminate drafts and give you 5-15 degrees more warmth just through the greenhouse-inside-a-greenhouse effect.
Wood is a pretty good insulator compared to nothing - a bit of plywood under a heat mat will redirect most of its heat upwards rather than downwards.
Dry cardboard might work, too.
P.S. When you put a heat source under a tray that has a humidity dome or plastic film covering, the heat will always evaporate more water than would otherwise have evaporated. If you have big drops of water condensing and running down the dome, leave the cover off for a while or prop it open, to let some of the excess water escape. Some fog is to be expected, and I usually have some small droplets condensing when the dome or plastic film is tight. But if so much condenses that it rolls off in drops, the soil mix is too wet.
