[stcredzero]

Like saying "therefore" might mean "I accept the premise".

I basically never hear that as a standalone reply in English!

As an interjection or at the beginning of a sentence, "donc", like "so", can mean "now then", "that said", "moving on" or "without further ado".

I'm not sure, but I think sometimes when my wife is saying, "schma" it's somewhat like "donc" you describe above. But sometimes, I think she expressing agreement with a person that a third party was a bit mistaken or off the mark, so it's expressing a kind of disagreement. Not sure.