I can't help with ideas about northern white cedar. I'm not even sure if it available here in Australia.

On the few occasions I have used WRC, I have found it flexible enough when I have used my heat gun.

I mostly use Paulownia and have not noticed that it is any more or any less flexible than WRC. It is about 20% lighter than WRC, so it makes the kayak about 10% lighter.

I believe that good Paulownia can be hard to find in parts of the US.

How much flexibility you need to make building relatively easy might depend on the design you are building. Some kayaks have more twista and bends than others.