[Ugaya]

Your party is well-balanced. You have all you need: melee fighters, archers and the magical/clerical support.
I never let a NPC join just because he comes with a quest. If you like your NPCs, just move on. Maybe after finishing the whole game you try another PC and different NPCs.
Regarding some of the missing quests: Korgan's and Mazzy's are (too) easy and quickly done, Edwin's can be fun (if you like Edwin, I found him disgusting), Keldorn's is laughable (but nice), Anonem's is interesting because of certain decisions to be made (he is unbearable though), Nalia's is o.k. Yoshimo's is special.
BTW, you will have the best (and longest) NPC quest (at least in my opinion) because you made the right choice.
It will be a tough decision to let Imoen join and dismiss one of the others...
A lot of fun playing SoA comes from the NPCs, their interactions, comments... I know, it's only a CRPG but I can't remember a pc game which is closer to PnP games.