Martial is very much the last stat to overflow, yes. The idea is that excess mystical beliefs first leads to more decorations and the like/excess art means more religious art everywhere. Once those are filled up it goes towards more trade goods.
For maxed out trade, first it goes into gaining more artworks from elsewhere to sell back home, with that art eventually overflowing into mysticism. Only once art and mysticism are overflowing does excess Diplomacy basically make the local polity so attractive that it starts drawing in more and more immigrants, producing Econ.
When Econ overflows, the first thing to happen is that the excess is traded away. Only once everything else is filled up do you start getting overflow into Martial from people lacking anything else to do.
Martial doesn't overflow, it just gets increasingly dangerous.