The US invading Canada isn't a realistic possibility. Any attempt to do so would trigger a civil war within the US, to say nothing of obliterating the geopolitical system. Attempting to annex in any way shape or form is currently nothing more than the musings of an old man, the only way any territorial changes could happen are through peaceful, mutually agreed upon transfer.

If the US were to seriously entertain the notion of invading its neighbor, 300,000 poorly trained reservists aren't going to seriously change the calculus. Canada is strong per capita but it has a fraction of the absolute population, military strength, and economy of the US. Nearly all of its major population centers are within extremely close proximity with the US border. It's military and economy are both heavily intertwined with the US, regardless of what rhetoric is being thrown around. A reserve force is for freeing up the active military to be used most effectively, defending key chokepoints, launching offensives, and operating complicated equipment, with the reservists doing things like preparing defensive lines, manning low risk areas, and supporting logistics. In a war between the US and Canada, the US would be able to launch large assaults with professional soldiers in multiple places with no natural obstacles over the short distance between their origin and objectives. There would be little for reservists to do to help - there would be no low risk areas to man, no defenses to prepare, very little in the ways of logistics to be concerned with. If Canada had serious concerns about the US invading, its money would be better spent on disentangling its armed forces from the US, acquiring counters to US systems, and establishing defensible positions between its border and major population centers.