once you are able to tank it dont really matter since in event like es/royoka you will be running a 2-3-0 formation
and unless you gona invest on dodge stone for yachiru which is pretty dam ex and it will get counter by hit rate i suggest you spend your time on sado since he a block tank will will pretty much last till you 80 or so
i would replace ichigo w/ renji asap b/c his stats arent very good, rangiku can also be replaced by momo (for pvp) and rukia (for pve)
yachiru is only better if u have free reishii to spend on her, sado is overall better b/c he has a higher block rate and block is definitely better than dodge in the long run