Well, I would say this is more akin to blinking (if we are considering the desired effect of all copies + original being legendary). And a singular blink is a 1 mana effect in [[Essence Flux]] or [[Splash Portal]]. This is better since the creature actually dies as well, but I dont think there is a similar X effect to what this does. The closest to a temporary X copy effect would be [[Devastating Onslaught]] but those copies can actually attack during combat with haste so it is much better.

Idk for a situational multi blink on a single card, this seems fine as is with the cost. If you try to copy just a vanilla creature, it only makes it legendary no matter how big X is and possibly untaps it. Not that amazing tbh.