I meant papers/documentation on FLANN.
a start:
- https://www.cs.ubc.ca/research/flann/uploads/FLANN/flann_manual-1.8.4.pdf
- GitHub - flann-lib/flann: Fast Library for Approximate Nearest Neighbors
and perhaps try AKAZE. you chose some very old descriptors there. AKAZE will certainly perform better, regardless of any other aspects you have to figure out.