as you mentioned the judge had read the entire document and upon that made his statement. Even if he cannot be charged within the legal definition of treason, it is very clear that General Flynn did not serve the best interests of his country and the oath that he took to the Constitution.

You do not betray your oath

of what he precisely did nor the UCMJ, I wonder if he could be held over for a Court Martial and have his pension reduced or revoked or some other punishment brought to bear upon him?

The federal district court and a court martial are both courts of the same sovereign - the US. He cannot be tried for the offenses that are part of his plea deal in federal district court. It is unlikely that he will loose his retired pay under the Hiss Act.

it was a thought.

but there is little chance for crimes not related to his military service. I don't think there is a legal impediment but it's just not done.

that he could make all that money and not betray his oath.

He didn't just break his oath, he sold it. Lock him up, forfeit all benefits, rank and retirement. But don't worry Pvt. Flynn, your room and board is still on us