I would suggest looking at:
https://dimaiolab.ipd.uw.edu/software/
for information on how to use rosettaCM in this situation, but I would use your phenix model to generate a symmdef file as described (if possible) and then run Either rosettaCM or RosettaES to complete loops. (I would run rosettaCM first, and then rosettaES on the parts you don’t like)
Once you have your symmdef file you should be able to provide CM with a single sequence, which will be modeled symmetrically.
If you are really stuck, I would suggest contacting Frank or me directly and we can maybe collaborate on a solution.