The first iteration worked well enough that I decided to make this into a proper product and integrate everything onto a 4-layer board; The DFM (design for manufacturability) is now complete and the PCB + stencil has been submitted to the fab.

PCB top view:

All designs uploaded to github.

I'm also doing a kickstarter to make this low cost VNA available to hobbyists:
https://www.kickstarter.com/projects/1759352588/xavna-a-full-featured-low-cost-two-port-vna?ref=8b36ci