I think having a corpse of the regular wither after you destroy the command block would be a good idea because technically the wither storms skin is technically a case protecting the phase 0 wither storm so it would make sense for a phase 0 corpse to fall out of the sky once destroying the command block

This doesn't really make much sense in my opinion, and also isn't very necessary. Also as the Wither Storm evolves, the phase 0 body amalgamates into the big black mass that is the later phases.