In Astarion origin playthrough we have it spelled out even better. This is what happens:
Cazador: The runes I carved into your flesh bind you and all seven thousand souls to the ritual. Complete it and those bearing the scars will be sacrificed - you included. Narrator: You can see from the glee in his face that he's telling the truth and he thinks you have no way forward. But then again, he didn't say whom the scars had to be carved into...
Basicly, if Astarion stands in the circle and starts the ritual he will commit suicide. If he uses someone else with the same scars as his placeholder then that person gets sacrificed in his stead, he gets to live and have all the power and simply keeps the scars as a terrible souvenir and reminder of his pointless in hindsight torture.
It's very clear. Astarion losing his soul in the ritual is just pure misinformation. As I said before, all the headcanons about the loss of soul and him being an abuser came from the spawn path. People just falsely assumed that when Astarion and the other companions told them he was saved and didn't become a walking meatbag then that's what automatically happened in the alternative path. Then someone must have spread the (holy) word, the parroting started and we ended up where we are now.
