So then with Astarion, it's better to just put the extra +2 Ability Improvement into his Dexterity which will give him a + 4 to ac and piercing damage
If that is the route you wish to go, then sure.
Remember, it is you playing.
Also remember we are just at Act 1 of a game that will likely have 2-3 more Acts.
At any rate, given Astarion already has 17 DEX, you could opt to go Weapon Master feat, which gives a +1 to DEX if you choose that, moving Astarion's 17 DEX to 18 DEX while giving Astarion more weapon use choices. Also, once Larian gets around to fixing it, you could also go dual wielder feat (which gives +1 to AC) with/for Astarion. There are other choices but those are the main two I use on various Rogue builds I have played.
Again, YOU do what you want but yeah, I suppose most players opt for the Ability Improvements +2 feat. *shrugs*
