One more note on this:
Depending on how you'd like to use DX-like FM synthesis, you should still consider the original DX7, as the Volca FM's velocity amount can't be controlled by an external MIDI controller. So for reasons of live playing expressiveness, the original DX7 and it's reincarnations would make more sense. However, if you only wanna use the Volca FM as a sequencer-controlled device, it's probably the better choice.
With regard to the DSI instruments you brought up it has to be said that the Evolver's FM capabilities are furthest away from anything DX-like, since it only offers exponential FM, which remains only musical (in terms of scales) at very low settings. The Prophet 12, however, offers linear FM in addition to that, which means that it can be used as a 4 operator FM synth similiar to the DX7. Keep in mind though that the DX7 and the Volca FM utilize 6 operator FM.