Realistically a scenario like that would have US/Can degrade over at least a decade if not more and it wouldn’t be some sort of surprise attack. Having a hostile nation to our south would prompt preparation and increase in numbers and equipment. Still wouldn’t be enough to stop anything but again how the world would react and who and public support in the US would be huge factors. A lot of states would not be happy with a Maga inspired invasion.
If I were Canada I would scatter the CAF into small insurgent groups, train locals and conduct small ops across the borders where we look and sound like them.
The US could easily control major Urban centres but maybe not the countryside so much. I would expect Canada to fracture into independent republics in reaction to it.