“There is really no reason to assume that genetic changes would stop at the point that it would affect the fertility of the offspring of a hybrid between a shi tzu and the original wolf population.”
It’s not an assumption. Despite decades of trying, with species of fly that’s life cycle is a week where mutation was induced by radiation and other means, and enhanced by artificial selection, true speciation has not been observed.
“The amounts of time talked about when discussing these things are so vast that it is very very hard to even begin to imagine them and the amount of time things could have had to change….”
But they aren’t so vast. The vastness is only from beginning to end but when different organism are viewed in isolation it’s not that vast. There is only about 2 million years to get from what we would today consider an ape to humans. That is about 100,000 generations (assuming shorter generations closer to the ape stage) of much smaller populations than we have today and much smaller than insects. Without induced mutation and artificial selection, 100,000 generations is not very much time.