I think you meant "be wp Expert".
However, first of all, why would you base your entire career on a single CMS?!
It's not even a framework. I agree that you can make money with it but, it's limited.
To create a theme, and only a theme, you just need to know the PHP syntax basic, to avoid syntax errors, that's it as PHP knowledge.
The rest it's about HTML and structure.
To create a plugin, you need to know PHP. Here it's not about "how much" because this is not a limited thing.
It's not like "you can learn everything but you don't know if it's worth it". You cannot learn everything about programming.
Just learn as much as you can, make experiments, create projects.
Does it worth it?
Yes, if you like it.
No, if you're doing it just for money.