Certainly interesting, I"ll be following this.
From what I understand of Class E, it would be difficult to have it remain efficient
across all of HF without some sort of switching.
Though just getting this to work Class C is a big win, with Class E on a favorite band or two.
FT8 is fine with Class E or C, does not need a linear amp.
Running an 800khz i2c bus into the si5351 is over spec by a factor of two,
Maybe one of the DDS chips would be more appropriate here.
Amazing this could be done on an Atmega328P.
Again, there might be better choices.