that said, he doesn't have any use of kido (i mean, he really sucks at stabilizing his reiatsu output) so he does need his superspeed and strength

On the topic of a zanpakuto, yes it is specifically for you and part of you and yet a seperate thing (a seperate entity in your inner spiritworld even). And if a zanpauto would be wieldable without drawbacks for size, why wouldn't all captains use a BFS? The fact that you can wield it with superstrength doesnt mean you can just swing it around like a twig (it seems even in the spiritworld some form of Newtonian physics seem to be present). I would've liked to see some more adaptation to different fighting styles for different swords. Just like the squad 1 lieutenant should use stabs instead of slashes, because he was a fencing sword. But whether a cleaver, a katana or any other type of slashing based sword, they all slash the same way regardless of size, type or whatever. Only if the weapon is radically different (liek a floret or combatknife or polearm) the style seems to differ...