It's easy to worry about gaps in your knowledge, especially when faced with 10,000 papers you haven't read. It's just as easy to fixate on this as the main problem and get stuck in a reading trap. Reading paper after paper, cramming as if it were an undergraduate exam.
Reading papers without doing any practical work is like reading a hundred cookbooks without setting foot in a kitchen. Even if you manage to absorb all that knowledge, it's useless without the skills to apply it.
Yes, you will be partly judged on your knowledge of the field, but they don't expect you to know everything. It's just as important to do your own research competently, and that takes practice.
Working on your skills is just as important as the reading.