a "peak detector" can be as simple as a resistor and a capacitor in parallel "smoothing" the output of the sensor, or like Jerson wrote, of the op-amp, right?
not really , it needs a rectifier too. if the wave is not symmetrical the neg and pos peaks may be needed. plus it still needs adc code to read it in anyway
May I kindly ask for a principle circuit to start with the correct foot please?

I don't see how to "rectify" a very low AC signal - I'm an electrician and used to easely handle voltages over 230VAC but when it comes down to millivolts.... 😬