And learning 1000 jutsu dont mean a thing for multiple reasosn. For one they could be weak justu or close range jutsu. Second with his sharingon on all the time he is never at full strenght so if the justu takes alot of chakra like his black fire jutsu then he can only pull one off which afterwards he needed kisame to carry him. Third if kisame does take his chakra away he wont have any jutsu to use, or sharingon.
About his sword. If might gai going gates cant break it what makes you think that itachi can. and even if he does he would of been close enough for kisame to take his chakra away. As well as saying that you think itachi can break it isnt a real valid statement. We dont know what it is made of for one. and have you ever seen itachi pull off an amazing feat of pure strength? most of itachis fighting ability comes from genjutsu or sneak attacks.
For the part about him killing his whole clan. most of his clan werent even ninja. His clan was known to be the strongest clan in Konoha. This could be because of their size not overall fighting ability. Kisame is part of the legendary swordsmen, they are known to be some of the strongest fighters in the nations.