The last phase (before the final DPS check) is actually the easiest one, with only a few mechanics. These mechanics aren't hard and don't requires the same level of awareness or tight execution that the previous phases do.
So if this phase was to be nerfed, it'd actually be too easy. In extreme fights like this, you often have either hard mechanics but with easy recovery, or easy mechanics with hard recovery. That's exactly what happens in Shinryu.
The echo and gear progression will be more than enough.