Larian breaks down the creation of Baldur's Gate 3, from memorable characters like Astarion and The Emperor to its various endings.
The other vampire spawn encountered, such as Dalyria, are level 11 when met in Act 3 of BG3. Their abilities differ from Astarion's, and he mentions that he was among the first of Cazador's vampire spawn. Therefore, it would be sensible to infer that Astarion was at minimum level ...