It's not the only option though. You can complete PoTD 15 times instead.
1st step - 15 memories from either Bozja CE's or 15 CT runs
2nd - 18 memories from either Gyr Abania fates or Mhachi runs + 18 memories from Far east fates or Ivalice runs
3rd - 15 memories from either Delubrum Reginae runs OR PoTD
Besides the 1 time for the story, you don't have to step into DR outside of your own will.