Alex, that would be a pretty huge delta. I'm not sure the scale of your landmass there but it feels big and huge deltas like that don't have a lot of real world precedents. That said, check out the size of the Ganges river delta which is absolutely massive so yea I guess it's surprisingly plausible. lol It likely wouldn't happen if it were not a wide, llow lying and jungly or marshy area as you said. The new lakes are cool and you could add some outlets to them too :p (We are outlet obsessed here, or at least I am) The two lakes on that western peninsula drainage basin are very similar in size and shape. I'd try to make one differently shaped, sized and possibly placed so it feels less symmetrical. Maybe even give one a second river inlet.