You don't seem to understand your own magic though. That is sensory illusion, yes. But this is a different type of illusion, this is literal transformation using illusion and disguise. You are going to be copying this magic basically, just because you 'really are it' doesn't mean its something new. You cannot break an Izkuthii's illusion just by 'seeing through their disguise'.
https://wikia.lordofthecraft.net/index.php?title=Izkuthii
Compared to:
"They must know exactly how their Illusion will sound, smell, feel, or appear. It is because of this that Illusionists must experience their own Illusions. If they choose to summon a tangle of snakes, they must have seen with their own eyes, ear, hands, and nose that tangle of snakes in the physical world. Due to this factor, it is extremely rare to find an Illusionists who can produce something like an ear bleeding shreek, because if an Illusionists experienced that noise in the physical world, they would most likely be deaf. After the step of creating the Illusion in their mind, next comes the step of actually making the Illusion come to be. Unlike the other sub-types though, an Illusionist almost entirely skips the step of drawing the Illusion from their mind and into the physical world. Once the Illusion is imagined within the Illusionist’s mind they must simply have the Illusion take place within the minds they choose. This is when the use of mana for Illusion comes in. By using their own mana to intermingle with that of their choosing (With or without the other person(s) knowing) they can reproduce this Illusion which is already within their own minds into the minds of others."
https://wikia.lordofthecraft.net/index.php?title=Sensory_Illusion