I agree that there is significantly more spellcasters than rogues, rangers and warriors. Lae'zel is the only warrior in the group. I feel like Larian should of added a Paladin to complement her but then again Paladins also have magic. Astarion is the only rogue but he's an arcana rogue by default. He should of been given mostly rogue attributes instead of giving him spells. Astarion should also be the only one who can picks locks unless you choose to pick ranger for your character or a rogue. As for spell caster companions, you have three, a cleric, a warlock and a wizard. I'm not sure what the abilities of classes are in DnD but I notice that Wizards and Sorceror have pretty much the same abilities minutes a select few. I also feel that Wizards should not be the only one that can learn spells. I think that is unfair to Warlocks, Clerics and Sorcerors. In my opinion, the game doesn't have to be 100% DnD just based on it is fine. DnD is based on existing lore, they just tweak it for their own. So, Larian can also do the same for a video game.
I figured that since they had added the Druid to the game the last update whose mainly another spell-based Character that they would add the Paladin this time as well instead of the Sorcer. true the Paladin does use some spells but so do most of the characters in the game like Lae'zel. But the Paladin would have been more combat-leaning like that of Lae'zel.
As for Astarion being the only one for which should be able to pick locks and disarm traps I would fully agree with you on this if it weren't for the fact that I can't stand him and I've left him in the camp this whole playthrough. He's to Whiny and girly sounding for me.
As for the Gale the Wizard He's been hanging out in the camp as well due to the fact that I don't like his stupid ritual you've got to go through to resurrect him if he should die nor do I like the fact that he has to consume our magical artifacts. Maybe one day I'll deal with him and Astarion with a couple of playthroughs its hard to say.
Wizards aren't the only ones for which can cast spells as you've noticed playing the game all classes so far can cast various spells whether it be for better or worse.
No my party right now consist of the Caina a female Ranger knight wielding 2 weapons, Lae'zel Shadow Heart and Wyll and so far we've been tearing things up. Shadow has the best AC at 21. The spells Caina uses is mainly defensive. I had one battle in the goblin camp fight the group with Dror ragzlin that I lost 1/2 the party and only Caina and Wyl's imp were left fighting.