Largest highway (four lanes all the way) that heads south and avoids most of the mountainous areas (until you get to the Kentucky area) is Route 75, which you could get either by crossing at Gananoque then going south on Route 81 to I90 West, OR driving along Hwy 401 to Sarnia, then taking I94 to get yourselves to I75.
It's a wonderful route, very scenic, and will even take you by Jackson Center, Ohio if you feel like visiting the Airstream factory, or even get something done there.
One thing to consider is that by taking I90, you will be buying fuel at US prices, which even given the parity of our dollar, is still w-a-y cheaper.
While it may look to be a longer route at first glance, in reality it's a much more direct route to TX than hugging the eastern seaboard. Look at the trip on Google maps, you'll see what I mean.
Enjoy your travel!
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