Why am I doing this?
You and I are a lot alike. Different in some ways (I'm certainly no Marxist, for example), but I can relate to a lot of what you say here. Atheist? Check. Anti-Trump? Check. Left of center squish? Check.
I will say that I am staunchly anti-"heritage" in the sense you describe here. You say that your responsibility as a descendent of a Founding Father is "debatable". Here's my contribution to the debate - to me it's anathema to the principles of individuality, which are foundational to Western thinking (sadly in decline on the American left). We do not hold people responsible for the sins of their ancestors. That's the stuff of blood feuds and ethnic conflict. As an American, I've seen enough of the deleterious effects of "white guilt" on society. Arguably it served a purpose for a time, but one needn't feel personally responsible for something in order to see that a wrong needs to be righted. And now it has become a toxic element of modern progressivism, suppressing what should be an uncontroversial opposition to a dangerous strain of illiberalism in leftist thinking. And I say this as a Euro-mongrel whose ancestors weren't even here during the time of slavery - so even though I'm white, I could easily justify an exemption from any inherited responsibility. But nobody should have to.
So suffice it to say, I don't think you should feel any responsibility for the sins of my country based on your birth. I think you have a far better justification in taking an active interest in America: you are a citizen of what we loosely call the "free world", and like it or not, America is ... *consults experts* ... yes, it seems we are still, more or less, the leader of the free world. Canada may not be perfect, but in many ways you set a good example for America - and believe me, it does not go unnoticed. So keep doing that, and learn from our mistakes.