If you should happen to own, or plan to own, one of those shopping "buggy" things seen often around the UK, that people use to bring home their groceries locally when use of a car is not available, you can usually slip the bag part off the dolly part and use that, too, as a dolly to wheel things around. I do that a lot with mine, even used it to dolly my boxes when I moved house here.
Edited to add: the only catch is, I don't know what you would do with the dolly if checking the large duffel into cargo. Check that in too I guess? Have the duffel strapped firmly to the dolly so that they are in effect of one piece?