Good question, I’ve only understood it as a difference of who speaks it and how thoroughly is it defined.
I don’t know if your example is one of an actual difference - the example I’d use would be:
Doctrine: Mary is co-Redemptrix, Mediatrix, Advocatrix
Dogma: Mary was Assumed bodily into Heaven
The difference is that the dogma has been investigated and defined
ex cathedra by the Pope, while the doctrine has been repeatedly described, even in Papal encyclicals, but has not received as thorough of an investigation or definition, and has not been pronounced
ex cathedra.
That said, there are dogma that existed as such prior to the first Vatican council’s ruling on papal infallibility, so I can’t claim that issuing a statement
ex cathedra is the sole defining line between doctrine and dogma.
And there’s a related point:
does mean we don’t have to believe the doctrine because it isn’t the highest certitude and subject to interpretation?
Technically you’re not supposed to dissent from either doctrine or dogma. I believe both are considered to be infallible teachings - relying upon the infallibility of the Magisterium, not necessarily deriving from the office of the Pope, so that Ecumenical councils and the CFD can declare or define them.
If I were cynical, I’d take the view I’ve found elsewhere:
“The difference between doctrine and dogma is this: if you have a problem with doctrine, shut your mouth. If you have a problem with dogma, go to Hell, because you’re on your way already.”
I wish I could find that page … would be helpful.