There are 2 things you need in to be op in One piece, in fact if done properly its possible to even eat multiple devil fruits.
You only need to get your hands on the technique Semei Kikan and Observation Haki, Semei Kikan is a technique that grants the user control over their body for more info just search it up on google.
You don't necessarily need Observation Haki, but if you don't have Observation Haki, try to gain knowledge on your bodies biology.
My idea is to use Semei kikan to enhance the body in every possible way permanently by manipulating the body to use up excess fat to be used as energy to make more muscle cells or remove myostatin which is what limit muscle growth.
Manipulation of your neurons which will bring a manipulation of electrical impulses can improve you speed and time perception infinitely as long as your body is strong enough.
The only side effect is the increased apatite.
The reason you need Biology knowledge or Observation Haki is so you can understand which part of the body needs manipulation or which parts of you body is a no go zone so you don't accidentally kill your self in the process. You could change your body how ever you want, you could even become a Giant, a dragon way larger than Kaido's.
As for the devil fruits, well i don't know personally I don't like the weakness that comes with it you might need to do a bunch of experiments but if you know what to do you can even make your body intangible by reducing body molecule density to become intangible like Obito kinda, that's only when you control of your body reaches molecular level.
There is many more to it but I'm tired and wanna sleep so ill end it here