Help for analog signal chain for 16bits ADC
The response of the whole chain got a strange behavior @1MH it come from the sallen &amp; key low-pass filter. I don't where the increase of gain came at 1 MHz and I don't know how to correct this.

I would step back and consider if that elaborate cascaded filter system even makes sense.  You have an ADC with a 3.6 MHz sampling rate and target bandwidth of 37 KHz.  Since you are oversampled by a factor of 100, the passive RC filter you put on the input to the ADC chip will give you 44 dB of alias rejection before you have even put a single opamp in circuit. You can then use a digital low pass filter and decimate to 96 kHz, or since it looks (if I understand correctly) like your MCU has hardware to do this automatically, use what it provides.
Do you actually need more alias rejection than that?  My guess is no, but a second order filter would give you more alias rejection than the dynamic range of the ADC.


NOISE SPECTRAL DENSITY OPTIMIZATION : CAN I OPTIMIZE ?


The ADC you're using only specs 83 dB SNR (13.5 bits), so I don't know that a lot of analog optimization is going to make a big difference here.  Assuming you don't completely screw up the analog front end, you are probably going to be limited by the noise on the ADC.
One thing you can try since you only need a small bandwidth is oversample like discussed above.  When you oversample,  your SNR goes up with 0.5*log2(ratio), and you're oversampling by a ratio of 100, so you could gain about 3.3 additional bits of resolution.  I would look into this.