I quite doubt you'll see such thing as sane leftists generally simply discount tankie talking points out of hand -- you look for certain patterns and the sources they cite and say "eh, not worth reading", and also "eh, not worth replying to" because talking to a tankie is talking to a TV. They have a closed world-view, are able to bend reality itself to fit their core beliefs in the same way hardcore flat-earthers can.

You'll be able to head over to random socialist places and find people who can readily address points, cut through the historical revisionism and selective reading that tankies do, but you'll have to actually, well, ask questions and things might get egg-headed. If you address tankies directly I recommend going for broad-strokes arguments and questions and refuse to let yourself be dragged into areas that can't be wikipedia'd quickly, say, the status of unions in the USSR. An unprepared tankie is not unlikely to flat-out claim that they were independent from the party. Ask them about whether they think people should be sent to Gulag over their interpretation of Darwin, suchlike: They will either deny it, at which point they disagree with Stalin which means that he did, in fact, do things which were not right, which is inconsistent with what they believe in, or they will support it, outing themselves as batshit crazy plain for everyone to see (and also disagreeing with post-Stalin USSR scientific community, much less the world's scientific community).

Coming to the question of "why are tankies the way they are in the first place" though we come, at least from an Anarchist POV, to vanguardism as a core feature everything hinges on: The idea that for radical change to be possible, the masses must be led by a revolutionary vanguard. Marxist-Leninists all tend to fall into that category in one way or the other (and there's plenty of e.g. Trots who are cringe but perfectly fine human beings) but it's tankies who take it up to 11 by declaring themselves (and of course Stalin etc) infallible, and any opposition to their exalted "infallible" positions as counter-revolutionary. Thus, if you are not of the exact same opinion as them you're the enemy and voila you have a cult going that can justify anything to itself.

This "change is caused by small groups leading broad masses" thing then leads to the "everything is a proxy war" type of thinking you see: It is inconceivable for tankies to think that Ukrainians would have a free will, a desire to decide their fate, and as they were drifting further and further away from Russia of course the CIA must be behind that. It's pure projection.

(And, just for the record, yes, even Anarchists technically form vanguards. From "farming commune doing its thing and writing revolutionary poetry" to "Let's stop right-wing militias from slaughtering native people and then live among the natives and talk about humanism", see Chiapas).

(Also, Tankiedom is a CONTELPRO programme, their purpose is to make lefitsts in general look bonkers and inherently oppressive. Convince me otherwise).