This is a comment not a question

I have lost count of how many talks I have been to on millimeterwave amps and I asked about harmonic tuning. The answer is always condescending, like, Steve, you dumbass, there's no gain at 2F0 so no need...

This paper seems to vindicate the idea. The highest efficiency amplifier at 30 GHz uses.... drum roll... harmonic tuning.

https://www.rfglobalnet.com/doc/design-and-performance-of-high-efficiency-ka-band-gan-power-amplifiers-0001