Regarding T3 skills, I too think that they're for the most part detrimentally broken for three reasons:
1- Their OHKO nature negates the differences in flavours their secondary effects may provide, like paralysis, sleep, halving speed/accuracy/skill... and unless the dmg or str multiplier of the ones negating def or res is reduced, it won't matter if the enemies are made less squishy. It's okay to have some be basic OHKO of different kinds (Astra is 5 weapon uses while Impale is a single use, etc).
2-Their same proc-based OHKO nature makes it difficult to balancedly introduce T3 enemies late in the game, even if it's just the boss of the map.
3- I, personally, really dislike the functionality and necessity of Nihil. If Eclipse and Aether weren't OHKOs (and also for Mantle but that's another matter) then Nihil wouldn't even need to exist. Or, if it did, it could instead have been coded so that two Nihils cancel each other out, but not the other skills (that is, as if neither unit had Skill). FE9's Ike vs BK was much more interesting because of the two skills' presence, and because single procs were survivable.
But for this to be changed, you'd need to study and figure out where and how scripts are made to work.
And isn't Sanaki's personal characteristic that she can't into staves, unlike all other T3 mages?
And as I can't post without thinking of something curious: Bane also inflicting poison. Now that'd make it unique and kinda funny: do you take the opportunity to do other things but lose the kill experience, or do you go for the kill but waste another unit's action?