I think you model is suffering from what I call “garbage in garbage out”.
you want your generator to generate new data from a randomized vector with no statistical relationship to ground truth? no way.
My suggestion is first to come up with a way to describe your ground truth as a vector of z_dim that is statistically relevant. Then feed that to your generator. Your generator will eventually learn enough prior so that when you do feed it a new randomly generated vector it can output plausible new data.
If you don’t want to do that. Then I suggest changing your Generator network to an Encoder-Decoder set up.