As I was dealing with questions about DnD characters in general - it became clear that Astarion just does not fit into 5e. Since 5e simplified everything, including stereotyping classes, he just doesn't nearly match the 5e definition of a rogue.
Now, under 3.5e he would have perfectly fit in as a rogue:
Rogues share little in common with each other. Some are stealthy thieves. Others are silver-tongued tricksters. Still others are scouts, infiltrators, spies, diplomats, or thugs. What they share is versatility, adaptability, and resourcefulness. In general, rogues are skilled at getting what others don’t want them to get: entrance into a locked treasure vault, safe passage past a deadly trap, secret battle plans, a guard’s trust, or some random person’s pocket money.
An aristocratic paperpusher dragged into vampirism fits well with a Lvl 1 rogue there - however, Larian did not choose to design the game under 3.5e, but rather they chose to use 5e - and I've figured out where I had the player's handbook for that:
Every town and city has its share of rogues. Most of them live up to the worst stereotypes of the class, making a living as burglars, assassins, cutpurses, and con artists. Often, these scoundrels are organized into thieves’ guilds or crime families. Plenty of rogues operate independently, but even they sometimes recruit apprentices to help them in their scams and heists. A few rogues make an honest living as locksmiths, investigators, or exterminators, which can be a dangerous job in a world where dire rats—and wererats—haunt the sewers.
Astarion doesn't have a criminal background; he has an aristocratic and vampire spawn background. 5e reduces rogues to criminals where a handful of them do honest work at higher levels - this does not work for Astarion at all. Either his class or his story must be reworked, because 5e is overly simplifed and reduced its classes to stereotypes.