I agree that something has to be done with DF in Civ6, and removing the possibility to buy or sell it probably would be the first thing to do.
However, as concerns comparison of Congress in 5 and 6, my feelings are completely opposite. I thought Civ 5 congress was the most frustrating implementation in the whole franchise, and in no small part because of its inevitable intrusiveness, made even worse by the most silly voting from the part of the AI. I wrote a few times about it on these forums, and here I go again. There was only one aspect I thought was good - the collaborative projects: World Fair, World Games and such. The rest was annoying garbage. What if I do not want to participate in such congress, who the heck enforces those decisions, why can't I defy some outlandish resolution? At least in 6 it is only temporary, and there are some resolutions that sometimes are quite useful.
However quirky Civ 6 congress may seem or be, I prefer it to 5 congress by a large margin and I don't want the impact of 5 to be brought back, no thanks.
Ideally, world congress membership should not be something that happens inevitably, that should be voluntary and it should bring some perks and obligations. And some possibilities to defy, and some sanctions for defying. But that would be rather tricky to code, I imagine.