Sallen key topology choosing right op-amp
The flatish response from 100kHz upwards is due to the rising output impedance of the opamps due to their low GBW product. Capacitors C9 and C11 rely on the amplifier having a zero output impedance. However, into a finite impedance, they allow a high frequency signal to feed forward from the resistor junction directly to the output, bypassing the filter.

There are three mitigations for this effect. The first is to use an amplifier with high GBW, and so high loop gain at the problem frequencies. The second is to raise the general impedance level of the filter components, so that the amplifier output impedance is a smaller fraction. The third is to use an odd-order Sallen-Key as the first stage, which comes with a real RC to ground, so taxes the amplifier output impedance less.

The shape of the plot shows you that even the AD8642 is short on GBW for the filter you want, if the stopband is important. Finding a fast, precision opamp may be expensive. It may be cheaper and easier to use a hybrid approach, so replacing each of U1 and U2 with a pair of buffers in parallel. Use a fast buffer with a low output impedance and no need for DC accuracy to drive the feedback capacitors. Use a relatively slow precision amplifier to buffer the voltage to the next stage.