Assuming you bandpass with 3dTproject (with the intention of removing at least slow drifts), I would still include exactly -polort 2, since sinusoids do not cover those degrees. And since polort 2 is only 3 regressors, I would not worry about DOF for them.
If bandpassing is only being done as a high-pass filter, it will not use so many regressors anyway. It is the low-pass filtering that devours DOF.
But indeed, be sure of what regresssors are being included (from fmriprep and from AFNI), you would not want to duplicate them. Do the fmriprep regresors include linear and quadratic terms? What frequencies are included?
- rick