Zener is slow => useless
Schottky => already in the ATmega so fixed that problem as well
Really? What makes you think quantum tunneling is slow? (Low voltage zener's usetunneling breakdown) In fact the high junction capacitance actually helps in a protection role.
However low voltage zener's have a soft knee, which is the show-stopper here.No, the internal protection diodes are pn-junction diodes, and capable of handling only tinycurrents (1mA or similar), so external schottkys will conduct first and can carry amps without issue.