They are pretty different. Narratively, for one, although you got dreams and special powers over the course of BG I, BG I is not a game in which you really have to face the consequences of your heritage. That doesn't start to happen until BGII. In BG I the struggle is against your half-brother Saraevok who is trying to murder you, but the narrative of 'fight what you are, embrace your ancestry' is very low key. Slayer doesn't even show up until BG II.
BG III the tadpole is treated as an immediate crisis that needs to be addressed in the here and now. A near and constant danger. You will die or worse if you don't get it out asap. The entire narrative push revolves around what to do with the tadpoles.
In the ruined Selunite temple you can get a conversation where the party members will remark after talking with Minthara where the similarities are pointed out between the party and the True Souls. It's even flat-out said at the time that the Nautaloid was probably going to Moonrise and that the party would have ended up like the True Souls had it arrived.
Omelum says that killing yourself might not be enough to get rid of the tadpole, so I am inclined to say that at the moment, the tadpoles abandoning the bodies of the other true souls upon death does look like a plot hole, until/if more information becomes available.
