I am gob-smacked at how much functionality Guido has been able to pack into the AtMEGA! I would think the cheap little
'blue pill' using the STM32F103 would be a much more powerful substitute. It has a very fast A/D, 128K of FLASH and 20K of RAM,
a 72 MHz clock and costs a few bucks. There's an excellent free C compiler (GNU ARM) and a good, free library (libopencm3).
I can imagine a really small, inexpensive SSB QRP rig with just a few components using his ideas and this controller.