The only reason Astarion even offers the spawn transformation (in my opinion, it's open to interpretation) is because he has a great deal of love for Tav and doesn't want her/him to have the option of leaving him.
This. I think the most realistic option for non-romanced PCs to become a spawn would be some sort of deal with Cazador. I’m sure he’d be open to it since he already has tons of living servants who only work for the possibility of future immortality, and he’d be fresh outta spawns after the ritual. Naturally, he’d promise that you’d be free and under no obligation to serve him afterwords (if you wanna take him at his word).
