It's outright nonsensical that one tadpole in your brain is a death sentence that will devour you from the inside out mind, body and soul... but putting more tadpoles in there doesn't rate a mention and I'm pretty sure no one ever mentions you have more than one.
I'm not entirely sure you're actually putting more tadpoles into your head. I know the graphic for obtaining new Illithid powers is you pick up a tadpole and drop it into a part of your brain for it to burrow in. But it doesn't make sense for multiple tadpoles to exist within a single occupant.
Also, usage of the first additional tadpoles suggest "Absorbing its power", in addition to the Astral Tadpole being usable multiple times (How would it burrow into your brain and still be available to burrow into your companions too?). We already have several mentions of the singular tadpole gaining in power (Ormeluum's probing annoys it and it burrows deeper unlocking the new skill. The Zaith'isk you can channel power into it empowering all Illithid powers to be bonus actions). So it's possible that the singular tadpole is just taking the power from the additional ones, which it then uses to do some (Directed) ceremorphosis.
That seems really easy! It seems like a simple binary. They end Act 1 and they have the tadpole or they don't. But then you hit Act 2 and, well, now you need to write a lot more Raphael content. And he got the tadpole he wanted from you, so, why should he even hang around?
Umm... He's a devil. Meaning he doesn't do a charity. He'd want something in return for removing the tadpole. Ergo, he has a reason to stick around. To ensure you fulfil your end of the bargain as well as to tempt any other party members who didn't agree to his terms (Wyll and Karlach for example would be extremely resistant to his offer).
Omeluum's Ring -- what happens if the player removes it? Why can't the player get the ring and then make a deal with Raphael? Is the player going to feel 'cheated' by having to give up an equipment slot for a narrative reason? Can they use the ring then remove the tadpole? Is the ring total immunity or only partial?
It's easy. The ring can mitigate negative effects of keeping the tadpole, much like if they had Raphael remove it. Without having to do Raphael's bidding. While still searching for a way to remove the tadpole like if they neither had Raphael remove it nor the ring.
If you really wanted, you could even write in some in-camp fighting over the ring as the story progresses and party members become more desperate to alleviate their concerns of their own tadpoles. Give it some LotR flair if you want.
Heck, tie in romances into it. Let you gift your partner the ring because you want to save them.
Keeping the tadpole -- is that fun for the player to get mind-controlled? What if the player gets to the end the Elder Brain just takes them over? Is that a possibility? If not, then why not? What if the player keeps it and never uses the powers? What if they keep it and use it all the time? How much are you willing to punish the player for a decision they made thirty hours ago, even as signposted as an bad idea as it is? If the player can keep the tadpole but resist the bad effects, then what's the point of the ring and Raphael?
Honestly, it could have been interesting to see various thoughts and actions try and take over your character, with associated saving throws caused by its mind-controlling effect. As far as using powers and the like... Using the powers shouldn't affect it since that's just you using your (Partially ceremorphed) brain. Gaining powers however, could make the aforementioned saves higher and/or more frequent as that is you furthering the ceremorphosis process.
Allowing for a better representation of "Do I give in and obtain these cool powers? Or do I stay pure and retain my humanity?" as opposed to the current iteration of "LMAO lets just take all the T1 powers at no consequence"
All in all, it's not hard to continue the concepts outlined in Act 1 and make them have a cohesive Act 2 and Act 3 core story progression. 2 paths still lead to "Look for a way to remove the tadpole" and 1 path would be "Follow the whims of Raphael" which could conveniently coincide with the other 2 paths (It borderline does anyway if Raphael simply sends you to Yugir instead of pretending to warn you away from the place. Just make up a reason for Raphael to send you to Gortash + Orin + Brainsly in case you don't confront him at the house of hope)
