Procedurally, Miller stands for the proposition that the second amendment is an individual right since Miller was successfully able to challenge the law all the way up to SCOTUS on grounds of a violation of his second amdendment rights.
Miller was not a "militia" member, yet SCOTUS reached the merits of the argument, therefore SCOTUS determined Miller had standing to bring the case. (Since Miller disappeared his side never briefed its case, so much of the Miller decision is based on only the government's arguments and is patently wrong - but that isn't the issue, Miller's standing is.)
Miller's procedural posture stands for the refutation of your position.
The prosecutor brought the case to SCOTUS, not Mr. Miller. Neither Mr. Miller nor his legal counsel appeared in front of the U.S. Supreme Court. Mr. Miller died before the decision was even reached.
"... on grounds of a violation of his second amdendment rights."
The lower court was vague -- they simply said the National Firearms Act violated the second amendment. They did not say why it violated the second amendment. They certainly did not say it violated Mr. Miller's individual right to keep and bear arms, as you claim.
"therefore SCOTUS determined Miller had standing to bring the case."
Again, the prosecutor brought the case.
The U.S. Supreme Court made no decision. They came to no conclusion. They never said the second amendment protected an individual right NOR did they say it protected a collective right.
They remanded the case back to the lower court with a question about the weapon's relationship to a well regulated Militia. Why would they be concerned about that relationship? What if the weapon had no relationship? What then?