Either they're a tankie and start singing the praises of the USSR and PRC and all sorts of totalitarian hellholes, or they start talking about hypothetical economic systems and states which haven't been shown to be practically achievable

Well allow me to present the third option: communism has been tried in Australia and North America, and it worked. Marx's ideas of what a communist society would look like were informed by descriptions he read of how the Haudenosaunee people actually organised their society. They did communism for thousands of years and it worked.