You see there are lots of games (especialy if you like certain themes, lets say it this way) ... where your protagonist is basicaly lost in the process ... same situation could happen here.
In Disco Elysium, once you start watching someone else's playthrough, you immediately turn it off so you don't spoiler further and get to play the game yourself as quickly as possible:
Hardspace: Shipbreaker is an example of breaking the illusion of continuity, when you first get to the place where you will be disassembling ships, an automated system puts you in a capsule and sort of kills you in the process of creating a digital copy of your neural network to revive in case of an emergency while disassembling ships.
Imagine:
You click on that dialogue option, offering yourself to turn ... and then you see your character squirm and being in pain ... next second, you are no longer looking AT the character, you are looking at The Emperor / Orpheus trough your character eyes, still squirming ... and the sight fades to dark in meere seconds, all you hear there is Narator telling something like:
"Too late you realize this is end of your journey, too late you understand true weight of your decision, but there is no going back, not anymore ... your body, your mind, even your soul, everything has ben lost in the process ... the ultimate sacrifice."
Then we let this nothingness for a second or two to sink in ...
And once again in first person, we would trough eyes of our character see in flash of light The Emperor / Orpheus abowe us ...
Narator telling something like:
"You were BORN anew! Yet you rememer your own death. You wonder for a second if is this really you, or just something that remembers you?
But there is time and place for such questions, and its not here, nor now ... you have work to do.
I imagined it... And... Impressive.

Yes, you can create a great transformation entourage from an artistic point of view.
Ha, I like it!

Still could work on the dialog lines - display the perception of this change in the dialog options, more other dialog options for Tav-illithid different from those of regular Tav, and more lines from the narrator relative to those of other characters - for example, another illithid could communicate with Tav-illithid mentally, and a dialog with a humanoid character may be framed by words from the narrator pushing Tav in a certain direction (“He's subject to his emotions and thinks this decision is absolutely right”, “You would have reacted differently before, but now you have a lot more senses, you're free of emotions and can see the bigger picture”).
In Disco Elysium, an example of conveying the perception of Harry's new personality is that the narrator, represented by Harry's different senses, says only what his new personality sees and perceives, and over the course of the game, as we unravel the events that led Harry to this state, also in parallel investigating the murder, by the end of the game we have a complete picture of who Harry was. But at the beginning of the game we get sort of a new identity for Harry, who is open to different paths.
If we're thinking about immersion, then yes, it turns out it is possible to create an impressive entourage of trasformation.