Wars aren't won with weapons. Battles are won with weapons. Wars are won with intelligence and logistics.
Russia tried to make their "special military operation" a battle and take Kyiv in the first few days. They failed, and now they have a war on their hands. If you follow the details of the war, a lot of focus is placed on cities that are well-connected to other cities by road or by water. Your military can be much more agile in where it chooses to deploy resources if you control the supply infrastructure.
Occupations are notoriously even worse. The asymmetry of maintaining resources for an occupation is huge. Relatively small pockets of resistance, well applied, can cripple an occupier's forces, even if the resistance is relatively poorly armed.
The question is what the limit of the American populous's tolerance for soldiers dying to occupy Canada, of all places. I hope we never find out.
I think we agree on the overall premise, but disagree on the degree. I also think that's fine. I don't know how hard it would be to arm Canadians broadly as you suggest. I'm suggesting that armament will be most effective built on a foundation of intelligence and logistics.
I think there would be value in something like the Swiss model (though I understand that it isn't as ubiquitous as it once was).