You bring up some reasonable points.
Argue the case. If you just did, it’s not a convincing one.
First, note I didn’t say long term residents of America. Just Canadian or Mexican citizens in general (be they long term residents of the US or not).
You may say it’s different. Maybe. Then just talk about those that aren’t long term residents.
The point here is equally true of citizens of Armenia, Vietnam, anywhere else (and I would say those that reside in America long term, or not) that aren’t US citizens. If they can’t vote in US elections, how can we consider the US a democracy?
Is democracy a binary? If so, is America one, or not one? At least is it closer to being one, or not being one?
Is democracy a spectrum? If so, where is America on the spectrum? Close to pure perfect democracy, or on the opposite end, close to the complete opposite of that? Or somewhere in the middle? You say its lacking. Where is that on the spectrum? What other countries are near it?
These types of discussion often happen around abstract, nebulous concepts like democracy, justice, good. Generally when they aren’t defined well by any participants.
If your point is the US could be more of a democracy, maybe. But even more so, it could be much less of one.