I haven't used Zephyr much, but I think NuttX is a smaller, simpler OS than Zephyr? NuttX feels like a great way to learn about the internals of smartphone operating systems.
I believe that is the beauty of it. Think something like the steamlink (streaming the video output of a steam game). Which turns into network transparent applications very quickly. Using a RTOS on the client side gives you amazing possibilities to lock the system down. Secure boot chains become a real possibility once you dont have to update that often due to the small attack vector. With NuttX having a simple framebuffer something like qubesOS but with remote VMs should be a real possibility. Especially since that is already happening with headunits in the automotive sector. Where one displayserver is handling both the safety critical tachometer as well as your multimedia stuff.
All of this not to mention, that the ARM processors in question dont have a secure coprocessor spying on you but Trustzone. This thread really gives me the warm and fuzzies. OP did something truly great here.