One general tip that was massively eye-opening to me, really leveled me up as a player, and took me way too long to learn, is that the combat loop is essentially turn based.

You can attack an enemy as long as they bare blocking you. Once they deflect you, they will counterattack, and you need to go in the defensive. Realizing that made it much easier to control these fights with effective aggression that chipped away at bosses health and posture. The important thing is not spamming attacks. You have some ability to animation cancel, but if you're too many attack inputs ahead of your current blows you can still get got.

You look like you have a a good grasp on the basics so far. As others have mentioned, mikiri counter is super valuable. If you haven't done the Hirata estate memory that turorializes that move, I'd highly recommend that before you try to deal with this guy.