Hi Garuspik,
Congratulations, great Project, I am developing a DIY Planar to, and thanks to open your projects to community.
I think its very smart idea to use a diamagnets bars, because it’s a simple way to guaranty a magnetic field without a metallic mask to provide a magnets short cut between different poles. If you want to improve your magnetic circuit my advise that you utilize FEMM software, it is simple and easy.
Other aspect of your project is the size of your membrane, biggest is better, better sound pressure with low THD, but in commercial aspect I think it complete opposite.
But I would like know, how efficient is your headphone, is it works in a mobile phone? Another thing is a bass response, because it is depends of flexibility of de membrane, a factor that is not easy to control, it is depends of de material that circuit is made, I think kapton in your case.
One biggest challenge for the planar manufactures is turbulence that magnetic at de front causes in air flow, but you ever comment that when you remove it, the sound lose the power. In my opinion this is the interest point to innovate.
Good luck.