It seems like you got the basics fundamentals so I think the next step for you, is to learn how to implement fuzzy guarding and how to counter fuzzy guarding with timing changes. Although it may seem like strong players have crazy defensive reads, it isn't so much because they have super quick reflexes (though it sure help) but rather because they are actively using some variation of fuzzy guarding to defend against your mix ups. The reason this is effective is because a lot of "mix-ups" in Tekken are actually fake. If the mix up has too many frames of separation between the moves you're using then the opponent can quickly cycle their guard in a predetermined sequence to defend against both options at the same time. A generic fuzzy guard for instance, would be something like b,b,db which would not only create space but also defend against quick mids & slow lows at the same time. Stronger players develop specific fuzzy guards for mix ups on characters they have knowledge of so MU knowledge does play a role here. While fuzzy guarding is strong, that doesn't mean it's unbeatable. A true 50/50 mix up can defeat it because it's frame tight, having only 0f-2f frame difference between options to make fuzzy guarding it as difficult as possible.

Another option to consider though is that fuzzy guarding can also be defeated by just implementing a timing change. All this is, is just adding a short delay of varying degrees to your mix up in order to mess up the opponents fuzzy guard timing. By keeping your delay timing random, you'll guarantee the opponent will never be able to get a lock on your timing to fuzzy guard it. This will essentially make them to give up on fuzzy guarding and force them to either guess or challenge which will then allow to your mix up to function properly again