I think astarion becoming a true vampire should be able to turn you into his spawn without needing to romance. it makes since for him wanting to start with the strongest he knows.
I agree, as his best friend, I figured my Character be the first, also not turning the Durge would be pretty dumb on Astairon part, he would have his own walking personal WMD, spoilers territory ahead
if I was Astairon having a Bhaalspawn Vampire at my beck and call would be an extremely OP weapon to put the other vampires in line, if some unruly upstart vampire tries to overthrow me as head vampire and I could send my personal attack dog in their full slayer glory after them
