I always play Astarion as multiclass Rogue.
This is how I split my levels:
3 Levels Rogue, Subclass: Thief (This gives you 2 bonus actions)
4 Levels Fighter, Subclass: Battle Master. Fighter gives "Action Surge," which allows the character to deal additional attacks. When prompted to get additional attacks, I select Disarming Attack, Menacing attack and one additional trait of your liking (I usually pick Rally or Riposte. Rally is so you can heal nearby allies)
5 Levels Ranger, Subclass: GloomStalker, select Misty Step and Silence.
You can level up Fighter and Gloomstalker interchangeably, it depends on if you want the extra attack actions first, or if you want the misty step first.
You can also do 3-3-3 into each of the above class, then allocate the remaining 3 levels how you see fit. OR even add in another Dex class build (Monk). This would allow Astarion to attack unarmed.
If you keep Astarion a Rogue, one of the smartest things you can do is MAKE SURE HE IS HOLDING AN OFF-HAND WEAPON. This is available at level 1. I can't stress this enough. If you have 2 daggers, equip this in his offhand. This makes him stab people twice in one turn, rather than just attacking once with his main hand.
As you level up, and are given additional abilities, I usually grab Initiative, Dual Weild and Tough for him. If you select Dual-Weild, he'll be able to hold heavier weapons in both hands. Make sure the two weapons are linked).
