I'm curious what interop difficulties you've run into in JAX? In my experience, the JAX ecosystem is quite modular and most JAX libraries work pretty well together. Penzai's core visualization tooling should work for most JAX NN libraries out of the box, and Penzai's neural net components are compatible with existing JAX optimization libraries (like Optax) and data loaders (like tfds/seqio or grain).
(Interop with PyTorch seems more difficult, of course!)