Using 3.3V or 2.5V for ADC and powering Op Amp from 3.3V or 5V
It looks like you are worried about voltage getting through to your STM32F.  Unfortunately, an SMBJ5.0A, like many suppressors, has a relatively high leakage current of 800 ua max.  This will introduce a large error in your resistor divider network.  You might try coming up with a circuit with the buffer amplifier on the unprotected side so that you can have a low impedance on the suppressor.