How left out down-sampling in 3D-DWT just on the Z direction
You are applying a 3D DWT scheme whose down-sampling is coherent across the three  directions. Nothing forbids you from decomposing bands the way you want, and the literature is full of so-called isotropic, Tensor, Separable, Square or Standard implementations (vocabulary: separable/non-separable,
standard/non-standard, S-form/NS-form, rectangular/square, anisotropic/isotropic, tensor/Mallat, hyperbolic/isotropic, separated/combined wavelet transform)

To do that, it is more appropriate to perform 1D wavedec passes your own way on each direction separately.