I had Astarion as Fighter-Battlemaster with Rogue-Thief during most parts of my Honour playthrough, and it is quite a strong build, at least with hand crossbows. A Sword Bard/Rogue or Sword Bard/Fighter or pure Sword Bard is (much) stronger, but that's mainly because Bard is so extremely strong and the most op items in the game are perfect for Bard.
I first leveled Astarion as Rogue until lvl 5, because Rogue is quite strong early game, especially with lvl 3 and the second bonus action. The combat you face at lvl3 and 4 and lvl 6/7 (Creche) is the most difficult in the game in my opinion. Then with lvl 5 I respecced to Fighter-Battlemaster, to get Action Surge, the second attack and the two weapons fighting style. I like to take all the feats, so I remained Fighter at lvl 6, from 7 to 9 I went Rogue-Thief. So with lvl 9 you have the second bonus action, giving you 4 attacks by default, and with lvl 9 you usually do the end fights in Act 2, which fits. In the end Astarion was Fighter 8/Rogue 4, to get all feats.
With the energy based hand crossbow you can buy in Moonrise Towers and the one you get from Yurgir in the temple Astarion was quite a beast.