I made some clones of the original mixer, and I am getting decent results up to 19 GHz. However, it seems like there is a huge resonance of some sort above that frequency which completely ruins my results. I am using 4 BAT24-02LS diodes (the cheapest I found was about $50 for 255 on winsource).
I accidentally broke the PCB, so I don't have any good pictures of it. But I have the gerber files for the board, as well as an image I took of the back side of the board before I broke it. The capacitors are 22pF 01005.
To test the conversion loss, I replaced a mixer in my VNA setup with my design. The VNA is uncalibrated, and the difference between the brown and blue line is the difference in conversion loss between the commercial mixer and my mixer. As you can see, my mixer is slightly lossier up to 19 GHz (which is partially explained by the fact that it is twice as large as the commercial design). However, after 19 GHz there is a huge drop in the mixer conversion loss, and I'm not exactly sure what is causing it. Changing the capacitors seems to have no effect. Touching the board also has no noticeable effect unless I touch the small wires connecting the diodes to the IF output. If I touch that, then the conversion loss drops even more after 19 GHz, so I suspect that I might have an unintended antenna somewhere with the pickoff for the IF. The wire which biases one end of the diode ring to ground seems to have no effect, and nothing happens when I touch it. In the commercial mixer, the wires are 5 mm long, but in my mixer, the wires are 8 mm long (so that could be a factor). Also I have no microwave absorber other than my finger.
Also please do not order the gerber files without modifying the design! The copper gap between the vias is too small and it will bridge if you try to get it manufactured. I had to cut the bridges open with a knife. I'm using 0.6 mm FR4 from JLCPCB.
In a later revision I'm going to add ferrites so the balun can work down to DC. My IF is 12 kHz (while the commercial mixer has to support up to 1 GHz IF) so I can get away with it.