Then setting ZSTD_CLEVEL in debian/scripts/install-firmware should do the trick (needs testing!):
ZSTD_CLEVEL=19 ./copy-firmware.sh --zstd "${staging_dir}"
Then setting ZSTD_CLEVEL in debian/ scripts/ install- firmware should do the trick (needs testing!):
ZSTD_CLEVEL=19 ./copy-firmware.sh --zstd "${staging_dir}"