Excellent - glad that’s working for you.
Keep buying boards from SparkFun, and everybody’s happy! And if you could say nice things about us on social media, that would help too.
Yes, you’ll need to process each batch individually. Look for those intervals of 50ms, discard the next two samples, and do your FFT on the remainder.
For the Uno R3, you’ll need 5V-3.3V level-shifters (your original issue).
I can recommend the Thing Plus C. It’s what I used for testing the code example: https://www.sparkfun.com/products/20168
Connect the accelerometer with a Qwiic cable, use a jumper cable to connect INT1. It has the microSD socket on the rear.
Best,
Paul