Taiga

well i think not really since you already have both karna and jailter so i guess you are fine+2 cu so she's not a priority

She's a very strong 3-turn burst servant, with 7 turns of downtime, and thats with her 1st skill at max level. If you like that kind of playstyle, she absolutely is. As for grailing, depends on you like her or not. I do.

Yes, she is one of the best 3 star lancers, only held back by her story lock. She serves a different purpose than the others that you have, so it's worth levelling her.

NP 5 Taiga is worth to raise and enhancing her NP level is quite difficult and she's a good Burst damage Lancer.

Chu is enough but he's leaning more into survivability, use Taiga is you want some bruteforce.