[Donthatme]

Here are my thoughts, as someone who completed a physics PhD not too long ago and switched academic fields.

There are two parts to your question: 1) How do I study up on advanced topics efficiently and 2) How do I do research with my current circumstance.

For 1), I would first continue working through grad-level textbooks. If you need some structure look at the textbooks used for required course at your favorite/dream grad program/school. These will hopefully give you a solid foundation of the physics/math for current research.

The next part about specific papers is tricky, because a lot of physics (sub-)subdomains are highly specialized and have their own terms and notations. In general though, my process is to look at the references and citations, and sometimes the references and citations of those. I am basically either looking for a paper or two that frames it in a way that I have seen before, or points to an old textbook that I am not familiar with. The process takes a bit of time at first, but is smoother after doing it a few times.

For 2), it depends on how deep into research you want to go. The current path would lead well to being well-versed in the current landscape as an outside observer. I think in all likelihood to do anything more than that you will need to have some affiliation with a research institution. If you want to be able to pursue your own research interests within those institutions, you will very likely need a PhD. Unfortunately it is hard to give more specific advice without knowing more details/desires. If you are in the bay area, you could also look at LBNL or LLNL and see if there is anything interest there for you.