I intend to make Astarion a thief/ranger and see no reason why it won't be excellent. Ranger 5/Thief 7 gets four attacks per round when dual-wielding, adding full attribute bonus and +1d6 hunter's mark damage on all of them and +4d6 sneak attack on the first. With short swords it's 12d6+20 per round (or +28 vs a favored enemy) before any magic item bonuses - nothing to sneeze at. A pure thief 12 would be doing 9d6+5. With Fighter you get action surge but lose Hunter's Mark plus all the ranger's other utility spells and abilities, which seems strictly inferior as a rogue multi. Plus the gloom stalker subclass gives a ton of other little advantages to a stealthy stabman.
I like this idea. Might steal it.
Although I am wondering who to mostly take along in my "core" party (alongside Shadowheart and Gale): Astarion with ranged spec or maybe Battlemaster Lae'Zal melee.
On one hand, ranged fire support would be very handy to eliminate the annoyingly spread ranged adds.
On the other hand, having a melee reliably push enemies off ledges and into (or back into) my Clouds of Darkness or Daggers (plus other persistant aoe niceties), as well as simply dominate simple enemies, would also be great.
Decisions, decisions. Guess I will split the game time between them.