GR is a done deal, theoretically. It's a theory of quantum gravity that's needed. Or, an alternative MOND theory.
The work you talk about starts with getting a degree in physics. Physics, like many science subjects, now has a huge body of established theory, and it takes 10 years of hard study to get to the forefront of current research. The same is true if you had wanted to prove Fermat's Last Theorem or develop a COVID vaccine. There's nothing fundamental left for a lone, uneducated student to discover.
Also, the sad truth is that until you understand the mathematical basis of GR, you have no understanding of it at all. It's like you've been hitting a few tennis balls around your back yard and you think you could beat Djokovic. It's impossible to play tennis like him without years of practice and dedication.