Given the overall reasoning for the Constitution, and the focus on the government working for the pople, not the other way around, the interpretation that militia is the State National Guard falls apart in light of the pivotal and central role of the people in determining their governance.
The text of the 2nd Amendment itself provides the key:
“A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.”
The only way it could interpreted that the 2nd Amendment refers only to the States’ National Guard / militia is if the 2nd Amendment is divorced from and judged separate from the fundamental meaning of the Constitution.
One of the most important discussions at the Constitutional Convention was the people having the ability and tools to overthrow a tyrannical government, just like the Founders did. In fact, they discussed quite openly that it is possible in the future the people may need to revolt again - they did not rest on their laurels once they won that all would stay free forever.
Since the Founding Fathers discussed the possible need for the people, i.e., civilians (which the Founders and colonists were), to revolt again, then exactly how would they expect the people to revolt if disarmed? Therefore, it is illogical to think that disarming civilians was a thought in the Constitution writers’ minds, as they were once armed civilians who gained their freedom through force.
Additionally, it can also be deduced that the 2nd Amendment involves the rights of civilians because the State government is what controls the National Guard. More accurately, the National Guard is also partly controlled by the US military, as its reserve units. Note that the National Guard is actually controlled by State Government and by the Federal Government and NOT by the people. In most stark terms, it is government bureaucrats who control and direct the National Guard.
Therefore, given that the Founders knew that, such as with their situation, government may one day need to be overthrown, it is absolutely nonsensical to deduce the Founders would deny American citizens the very route and tools, i.e., firearms in the hands of the people (civilians), that they used to gain their freedom.