Calculate interferograms for images pairs of different dates (all at 6 day intervals at best) and stack/coregister them. You can then calculate the phase average in the band maths. However, I am not sure how the snaphu export will recognize this band maths result as the correct image to export and process within snaphu, because usually the input for the snaphu export has i, q, intensity, phase (and sometimes coherence). So we would need to find a work around to replace it by the averaged interferogram. One solution could be to perform the snaphu export on a “normal” interferogram and then substitute the exported img by the average raster. But the coordinates have to be the very same.
I have seen the answer of johngan, he basically suggests the same. But his post doesn’t include the unwrapping step (as far as I understood it).