How to avoid the "need" to learn more before conducting research?
I sympathise completely with your situation, as I went through exactly that thought process when I began my PhD. I am in my second year.

I did not begin my PhD with a tightly defined research question, and I would guess that you are not doing so either. I did have an idea of what general area of research interested me and spent a lot of time looking at  published papers. Sometimes, while reading a paper, I realised that I needed more general understanding of that topic, and then read text books, or attended classes. 

I also attended seminars and such like, to get some idea about how other researchers in my general field go about their work. And I volunteered to give talks on quite general subjects in my broad field, on the basis the best way to learn a subject is to try and teach it to somebody else. I have been surprised by how much of that haphazardly gathered learning has turned out to be helpful in my own very specific research - sometimes directly relevant but more often by suggesting ideas to me about a way of looking at a problem.

I thought, and still think, that it would be a mistake to take classes and read textbooks until I was at the frontier of knowledge, because I would never reach the frontier that way.

What I did instead, after reading, more or less deeply,  a few hundred papers, was to realise that none of them really addressed a particular question that I was interested in. Having, almost inadvertently, identified a specific research question, I could then conduct a very intense literature review to confirm, or otherwise, that nobody else had researched that question.

Guess what! I had reached the frontier of knowledge. I am now in the process of seeing if I can push the frontier further out.