I think people generally mean "state", but in the US-centric HN community that word is ambiguous and will generally be interpreted the wrong way. Maybe "sovereign state" would work?

As someone with a political science degree whose secondary focus was international relations, "Nation-state" has a number of different, definitions, an (despite the fact that dictionaries often don't include it), one of the most commonly encountered for a very long time has been "one of the principle subjects of international law, held to possess what is popularly, but somewhat inaccuratedly, referred to as Westphalian sovereignty" (there is a historical connection between this use and the "state roughly correlating with single nation" sense that relates to the evolution of “Westphalian sovvereignty” as a norm, but that’s really neither here nor there, because the meaning would be the meaning regardless of its connection to the other meaning.)

You almost never see the definition you are referring used except in the context of explicit comparison of different bases and compositions of states, and in practice there is very close to zero ambiguity which sense is meant, and complaining about it is the same kind of misguided prescriptivism as (also popular on HN) complaining about the transitive use of "begs the question" because it has a different sense than the intransitive use.

It sounds more technical than “country” and is therefore better

To me it sounds more like saying regime instead of government, gives off a sense of distance and danger.

Not really: nations state level actor: a hacker group funded by a country, not necessarily directly part of that country's government but at the same time kept at arms length for deniability purposes. For instance, hacking groups operating from China, North Korea, Iran and Russia are often doing this with the tacit approval and often funding from the countries they operate in, but are not part of the 'official' government. Obviously the various secret services in so far as they have personnel engaged in targeted hacks are also nation state level actors.

It could be a multinational state actor, but the term nation-state is the most commonly used, regardless of accuracy. You can argue over whether of not the term itself is accurate, but you still understood the meaning.