both pretty extreme

Tankies don't even really fit most definitions for leftism that try to use something more concrete than vibes. They just think they're far left because they like the aesthetics of governments that tried to be or at least called themselves communist.

If we're using the original definition of left and right, they'd technically be on the right.

The original meaning was whether or not you supported the monarchy. I'd say that a dictatorship is close enough that it applies.

Of course, politics isn't one dimensional. Even the "political compass" isn't really enough, here, there's probably an axis of the political graph for each major axiom of governance.

Honestly the best descriptor for tankies is just "authoritarian communists." That tells you where they stand better than any attempt at a spectrum or graph.