Time to add some balance:
Above — A mixer with differential RF and IF ports. Some tuning on the output serves to further suppress the LO and RF at the IF port. I applied discrete, matched transistors, but sometimes build them on a CA3046 BJT array for even better transistor matching.
Above — Analysis of the BJT mixer output. The LO and RF are roughly down the same power from the desired sum IF @ ~20 MHz. (LO and RF = 36-38 dB down). From my real-world balanced mixer experiments, you’ve achieved reasonable balance when the LO and RF are >= 30 dB down from the LO & RF sum + difference frequencies.
Although usable, I realized I wouldn’t have enough room to build all the needed filtration plus amplification circuitry and thus went to a tiny MCL TUF-1 diode ring mixer.
To run this diode ring mixer, I changed the buffer on the L-C VFO to that already shown and adapted the ever-evolving 16.91 MHz xtal oscillator circuit to chop the TUF-1 LO port with a solid 50 Ω termination @ ~ 7dBm.
Above — Low noise, low distortion, well buffered 16.93 MHz crystal oscillator.