Just buy a thermostat controlled plug and put it in your tent hooked to a small heater. It probably won’t run much with lights on. You’d be surprised how much a grow can warm up your garage even without a heater. You don’t need too much air exchange early in the grow so just set your ventilation fan to come on over like 77f. Once the plants get bigger you’ll probably need to set your tent up to vent more often, or you could just buy a co2 tank and not worry about air exchange and just work on humidity management.