I actually share general skepticism about the US-Canada border and the structure of the Canadian state generally. But I also have deep skepticism of the US project generally.
I think northeastern US states have more in common with us than their own southern states. I think the Canadian political class -- both conservative and liberal -- are really parasitical awful people overall, and our own business community are oligopoly-trending douches with a penchant for using regulatory capture to screw their own citizens. I think Quebec could as easily be its own nation, in a north american federation and that the structure of much of the Canadian state is arbitrary.
But I think you've touched on something, which is that Trump has poisoned all discourse. I like many others have turned rabidly nationalist in the last few months.
In any case there's a reason why Canada exists. It's not some accident of history or just some retrograde unenlightened loyalists who liked the monarchy. Many of our ancestors saw aspects of dysfunction and injustice in the way the US was taking shape, and chose Canada as an (imperfect) alternative. And that there is an "alternate path" for governance in North America is in fact I think the precise thing that actually enrages people like Trump.
As for the "who is us?" and the invasion comment, my point is only that there is actually a long-running "meme" inside American politics since the very foundation of the US that objects to the existence of Canada at all, and included the assimilation of Canada in a large Manifest Destiny project. It's usually been a fringe position, but it has at times become amplified. E.g. under McKinley there was similar talk as what Trump is mouthing now, and of course during 1812, etc. It's "out there", but it's consistently present.
And that's the reason Canadians take this annexation talk seriously.