Bamboo requires Aluminum size diameter for optimum stiffness. Yours looks more like a steel bike.
I've never had a problem when using bamboo of larger size for at least the downtube.
The necessity of stiffening the rear triangle doesn't seem to be a problem. Short pieces of bamboo are virtually flex free. I shorten the chainstays as much as possible. The frame I posted has a rear triangle that's borderline too stiff.
I do select slightly thicker walled pieces for the drive side of the triangle.
I'll get some pics up of my latest.