I attempted it and lasted quite a while (don't remember how long exactly), until I made my rivers quite a bit slimmer - they take up so much space! Overall, I think I've preserved most of the original shape of things on my island, just a few minor adaptations here and there :)

I just looked at your map again and wow, your cliffs are tiny! Also getting to Redd's ship might turn out to be pretty inconvenient, lots of climbing up and down ladders

Maybe you can "cheat" a tiny bit and make a fake bridge? So terraform across a river and put some custom designs on it to make it look like a bridge...idk how much that would help with the overall situation though, seems to be a very difficult island to do this challenge on!