Currently the code which calculates the Butterworth coefficients and implements the filter only covers 2nd order – I need to do some more reading / understanding to go higher. I suspect 4th order at 0.2Hz might be better. Cut-off and sampling frequencies are already configurable.
The accelerating drift to the right suggests calibration errors rather than a take-off problem which normally leads to a fixed velocity drift. Calibration cube out again, I think.