Kanako is very disappointing, yeah. Her MND is equal to that of people like Maribel, who are not known for good MND, and her DEF isn't really any more exciting. Her HP is fairly low, so she really can't go in the front row to use her DEF anyway. And then her damage is fairly ehh. My Remi does more damage against Yuuka then Kanako's Suiga does, and Yuuka is weak to CLD! ...I got out my 2nd playthrough save to check this, and this isn't a party I really think is good anymore
Remi is jeigan, yeah, she doesn't scale into Plus as well. An ATK-based Remi will do nice damage still, especially considering good spd/delay, and ATK-based will still hold the 2nd slot great, or at least be quite good for 3rd. She isn't really awesome or anything, though. A DEF-Based Remi would probably be kinda disappointing in all aspects instead of okay damage+okay tanking.
You do have to keep in mind that damage scales differently in Plus. For example, Meiling will actually deal nice damage with Mountain Breaker. Enough to want to use it when there's absolutely nothing she needs to do. 80k when your other attackers do 200~300k isn't bad for a tank if they have nothing else to do. Besides, you need to buff her ATK for her heals. In the same way, Eirin's damage will improve in usefulness for Plus (Although it'll still suck unless it's a MAG-Eirin which isn't a good idea IMO)
Yuyuko, she's good once she has SP to spam SFN. Don't use her before that. Seriously. Even then, she'll be useless for Serpent of Chaos and Shikieiki; but other then that, she should be great once she has that SP. Someone you could consider getting after beating SoC; her MND/HP is pretty nice for a nuker. But yeah, the fact that she'll be useless on SoC really hurts her even once she has the SP to spam her nuke. And she's no good at all without SFN spam.
I'm wondering if I should use Renko or Iku or Keine or simply only Ran, for my offensive buffs this game... Ran would really only be "okay I'll boost your damage slightly... after using like 400 SP and several turns..." because god that's some slow buffing. It's very good, yes, but I'd really like a faster alternative.
Or I could use only Ran, but think about getting self-buffers like maybe Maribel (because I like her but she's really kind of... well, as the wiki says, an Extra Effort character. She's just a little behind other damage characters, but not by much, and can buff her own DEF/MND/SPD too, so.)