http://web.mit.edu/6.111/www/f2017/handouts/FFTtutorial12110...
shows an FFT implementation using a Cyclone EP2C20 FPGA. It can do 50ksps, 1K points, 12bit precision, 100 µsec/1K complex FFT @ 50Khz clock. May be a Zynq can be used to build a standalone device?
I wonder if the EEG signals can tell if your attention wanders. That may help improve meditation.
Have you looked at/played with the Muse or Muse2 EEG headband?