However, considering to really become good at this game you need to be able to be adept with no less than 6 characters,
What do you mean by that, out of curiosity?
I'll give you the stuff you said, but to me, Kensou's lame out really isn't just fireballs, I feel like most of his fighting is done at about the tip of his sweep range until he can get them to hold still for an A fireball, in which case he can run in and do mixups. Otherwise it's just looking for stuff to DP because IMO he has one of the best DPs in the game, since it's done from back, even if you can't get it out in time in response to short hops and stuff, you still block, so it's almost like an option select. That in itself eliminates a lot of the risk in going for a DP against jumpers. So if you eliminate their ability to jump, then they have to deal with Kensou's sweep, and most characters really don't have the pokes to compete with that, which leaves them to guessing short hops (and eating more DPs)
But yeah, A lot of characters DO have answers to fireballs, but there should be a kensou right behind the fireball when you throw one, and they may not have an answer to that when you're attacking in tandem with it. I don't think he's the kind of character you just throw fireballs with. Might as well play ash or saiki if you want to do that.
Anchor definitely isn't the optimized spot for him, but I think he can work it when he needs to. Second is probably... IMO, the best place for him. You'll have enough meter to do what you need to do but the opponent won't have so much of a surplus that it's a big issue for Kensou.