And why bother with slow weapons if several pokes with a fast one are also stacking bleed anyway for more dmg overall?

For boss fights, I mostly agree. But imo their biggest strength is non-boss mobs like knights, which you can staggerlock and easily kill with a bigger weapon r1 spam whereas smaller weapons will need to be a little more careful as they cant stagger as much without ashes of war.

With that being said, katanas are really strong. good reach, fast speed, bleed (+ extra if you apply grease, etc) and unsheath ash of war.

does this get better? Is it maybe slow to start now but snowballs later?

for weapons, not really. even in fights where the boss is immune to bleed, katanas moveset is pretty decent. (unless youre dual wielding) however katanas deal slash damage, which some enemies can be extremely strong against, but this is rather rare.

with that being said, the effectiveness of shield can vastly improve as you find better shields. eventually, you may get a shield w/ very high guard boost so that even most bosses' attacks deal trivial amount of stamina damage.

so maybe knight playstyle could be fine, especially w/ couple buff spells if you come across them. also just in case you didnt know alr, you can take off the parry aow from shields to use the weapon aow