Actually, because others have been in that same situation, fat_proc_convert_dcm_dwis will do the same conveniences of centering the volume, making it a nicer orientation (RAI), and making images, but starting from NII and bvec/bvals, as if the dcm2niix(_afni) part had already been run:
fat_proc_convert_dcm_dwis \
-innii study/subj-001/acq_piece_0*.nii.gz \
-inbvec study/subj-001/acq_piece_0*.bvec \
-inbval study/subj-001/acq_piece_0*.bval \
-prefix study/subj-001/dwi_all
-do_movie AGIF
--pt