Quick update to show how everything is going. Ignoring the river doesn't drown you, collision bugs etc, I've been pressing on with getting the major features in. There are now menus, UI (basic to start), game state logic, winning, losing, and just today I was putting in sound.
Sound has been quite tricky because 3d sound and listeners seems to be currently broken in Godot (3.05 at least), so I've had to bodge around the broken bits considerably. If they fix it I can solve some of these things but for now sound will have to be pretty simple.
Still loads of stuff to fix, as well as putting in some kind of progression as you complete levels. I was thinking of making roads / rivers wider, or multiple versions with more traffic, different speeds / densities. Also I might put in some other stuff like a fly to eat and otter and snake, depending on time.
I also figured out some performance issues were occurring because Godot was defaulting to a 4096x4096 shadow map (!), gawd knows how this was decided to be a good choice lol. 
Great progress so far.
How are you liking the engine?