I agree with you right until the end:

This is closer to the end of the Roman Republic — and strange events such as threats against Greenland and Canada can be interpreted in that context as well.

That is, we’re not seeing the downfall of the US empire but the downfall of the US republic that will eventually retreat to North America and retrench its position — hence moving Canada and Greenland (and Cuba and…) from informal relations with a hegemon to formal vassals of an empire. And emerge as an actual empire, much like Rome before.

While I understand the emotional temptation, using the word “empire” in place of “hegemony” can lead to inaccurate predictions — eg, total US collapse versus pullback for retrenchment and reformation.