When is one 'ready' to make original contributions to mathematics?
It is something of a myth that everything has already been studied and that you have to master thousands of pages of prior work before you can contribute something new.
To be sure, there are some subfields of mathematics that are highly technical, and you're unlikely to be able to contribute something new to them unless you've studied a lot of background material. However, there are also areas of mathematics that don't require that much background knowledge. For example, Aubrey de Grey recently made spectacular progress on a longstanding open problem in combinatorics, and almost no background knowledge was needed for that problem. Even in supposedly highly technical areas of mathematics, people sometimes come up with breakthroughs that employ very little advanced machinery.
As others have mentioned, more crucial than "knowing everything" are (1) finding a good problem to work on, and (2) having problem-solving ability. If you have both of these, then you can typically learn what you need as you go along. When you're at an early stage in your career, finding a good problem generally requires an advisor, unless you have the rare ability to smell out good problems yourself just by reading the literature and listening to talks. Problem-solving ability is probably innate to some extent, but a lot of it comes down to experience and persistence. Of course you will be a more powerful problem solver if you have a lot of tools in your toolbox, but generally speaking, you get better at solving problems by spending your time directly attempting to solve problems, and only reading the 500-page books when it becomes clear that they are needed to solve the problem you have in mind.
Mathematics is not learned by reading books. One becomes a research mathematician by solving problems. Most people need an adviser to recommend a good problem. Then you start thinking and reading what is relevant to your specific problem. General education by reading books with hundreds of pages can be done as a parallel process, but the main emphasis should be on a specific problem. It is a duty of the adviser to find a problem which does not require too much reading.
There are many examples that demonstrate these principles. Many good mathematicians obtained their first original results before the age of 18 or even much earlier, at the time when they learned very little.
Myself, I published my first paper at the age of 18, when I was a second year undergraduate student. I did not know much of mathematics at that time. I do not say that this paper is among my best, and at present I would not publish such a result, but this is irrelevant. The main point I am trying to make is that one has to solve problems, not to read books. It is not necessary that problems you solve in the beginning are new/publishable. But eventually you will obtain new results. Finding a good adviser is a crucial matter, for most people.
As a partial answer, learn to trust peer review. When you are starting out in graduate-level mathematics you focus on proofs and seldom go beyond statements which can be exhaustively reduced to basic axioms. In some ways that is the ideal of mathematics. But, when you get to the research frontier, you might discover that you need to use a statement which is contained in paper A, which at a crucial step in its proof invokes a result from paper B, which in turn invokes papers C, D and E, ... It could perhaps take months of work to see how that single statement ultimately follows from what you currently know. If you plunged down every such rabbit hole you encountered, it is unlikely that you would ever make progress. Exhaustive background knowledge is not a prerequisite for the creation of new knowledge. You can explore what follows from what is currently known, without first reducing what is currently known to first principles.