The reason you would turn Renko into Transcendent instead of Guardian is because
A.You get a percentage damage reduction on Transcendent already, so you don't miss the guardian's
B.The base stat increase is gigantic compared to the one you get from guardian, and for someone with maintenance, that makes a BIG difference. When I made Nitori transcendent I think her def/mnd increased by about 20%- it's not quite as drastic on Renko, whose starting base isn't as low, but it's still very significant.
She can't make use of the damage increase but it's not like it's "wasted"; you generally aren't really using several of the tools a subclass gives you anyway. How often does anyone's Enhancer actually use their subclass skills? (Okay nevermind, my Yukari used the atk/mag buff sometimes and earlier in the game I had Enhancer Sakuya to help out since buffing wasn't easy yet and extra attack is cool)
Also, the best affinity gear later in the game (not counting the +100 for rare specific situations) already boosts your def/mnd by large quantities, so you get them both at once. I guess if you wanted to use the Guardian's def/mnd buff skill there's that, but in postgame the only reason you wouldn't be using Byakuren is because you just really don't like her or to challenge yourself. Charge is still useful but you wouldn't really bother burning spd/MP to refresh the def/mnd buff, is what I mean.
Efficient Concentration -is- kind of nice to use Charge more, admittedly (Easygoing's heal is minor if you don't build for it, it's still a nice addition but) but you probably won't need it too often regardless due to Byakuren, especially since on a non-Enhancer Renko it really hurts to use it. I'd call Guardian a distant third option that's still got upsides, but not a particularly worthy one compared to Enhancer or Transcendent.
Also, yeah, Nitori is just crazy, given she'll become as tanky as your tanks in a FULL ATTACK BUILD and has one of the strongest attacks in the entire game, with the gigantic mp recovery potential to spam it despite cost and delay. I wouldn't say Renko exactly makes other tanks obsolete since lots of other tanks have special qualities and her moveset is limited and begs for certain subclasses unless you just ditched it for Pharmacologist (viable but lol) and, quite honestly, you get Renko SO LATE IN THE GAME that I'm kind of glad she's got enough of an edge to shine a little. Maribel is kind of meh other than her passive to treat enemy buffs as debuffs, but she's quite versatile at least and many late postgame bosses buff themselves sometimes so I didn't feel like I wasted the slot... but yeah she's generally pretty eh.