Is the ESD and overvoltage protection for my ADC circuit sufficient?
The most important component that needs to be there is R1 and it is 100 k ohm so you're good to go. Any IC including the AD820 will have ESD protection diodes at it's input already. You added additional BAT54 diodes in parallel to them, that will not hurt but they're not really needed either.

Those internal (on chip) ESD diodes will protect the chip sufficiently as long as you limit the current, which you do with R1.

The TVS D2 will only offer additional protection as long as there is something (like series resistance) present to limit the current. If the input voltage is enough to make D2 conduct but the current itsn't somehow limited, D2 will self distruct.

So: all in all your schematic looks fine to me, you could remove some components if you want to save cost but it will also work as it is now.