With a high source impedance the signal will be distorted, but this may be actually not that bad. The rectifier may still work. It is just that the signal one is looking at than is not longer the input AC, but something in between.
How much the capacitance matters also depends on the signal impedance. With higher source impedance it gets more important.
For the torque sensor it many still be better to use phase sensitive detection, as this also captures the phase and thus gets a sign to the amplitude.
The simple rectifier with diode gets tricky if the amplitude gets much below 100 mV as it gets no longer linear. An active rectifier with OP is comparable effort to phase sensitive rectification, possibly even more tricky with the relatively high frequency.