Tell us what a non-authoritarian leader of Venezuela would look like

Presumably they would look not-authoritarian, a description that doesn't fit Maduro at all.

It could well be that, in the face of US policy regarding Venezuela, only an Authoritarian could hold onto the country. That still doesn't make Maduro not an Authoritarian.

it seems to me that leftist leaders are always denounced as authoritarian by North American and European based NGOs and governments.

That's a fair observation but, again, that doesn't mean they are wrong when they say it about Maduro. Maduro is referred to as dictator by Human Rights Watch, the Organization of American States, and other human rights organizations, including some inside Venezuela.

Maduro is a dictator. It's largely the fault of the US that Venezuela has a dictator. If the US succeeds in ousting Maduro, it will almost certainly replace him with an even worse Dictator. All of that can be true with no contradictions.