Hi
Please try matroskamux plugin. It does not require Eos for this file container. Please use it and see if the video file has bytes and can be played.
And here is a C sample for reference:
GStreamer freeze when using qtmux and NVIDIA-accelerated h264/h265 encoding - #7 by DaneLLL
Not sure but it looks like in your code, you send EoS but do not poll it. You may add the function call same as gst_bus_poll() in python and try.