Just a small comment:
It is possible to design a VNA. I did it, but it took me a year of working fulltime . The VNA worked form 10 MHz up to 1.5GHz, I designed the RF part from synthesizer to receiver to testset to measurement algorithms. Clearly I learned a lot from that experience, but was it usefull apart from that? I doubt ...
VNA's are designed by very large teams of experienced and seasoned RF designers. Surely you can design a VNA by yourself, but do not expect to compete with their specifications. I don't think you'll have the same budget as them, so they cerainly will be able to use components with better performance.
If you do it, so it for the experience, but it's not a design made overnight.