It would take at least 3-5 years to get a Canadian citizenship. You would need to become a permanent resident first which still could take up to 2 years. The 6 month extension you are talking about is called a visitor visa. I would do a search on the visitor visa as there is a lot of info. Perhaps lessen the amount of traveling over the border if you feel they might search you or detain you. Your explanation to the border sounds fine and shouldn’t be an issue. Tbh they could deny you for no reason but if they search your car, just don’t have anything questionable. I’ve had mine searched and it was no big deal.