I would say "donc" is used where some might say "so" in English (aka shorter and less pompous "therefore").

I think the meaning of "Agreed" or "Right on" is rare and more implicit. Like saying "therefore" might mean "I accept the premise".

As a question, "donc?", like "so?", can mean "and then?", "then what?", "meaning what?", "what next?" or "so what?"

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".