I am going to answer this question from the perspective that you are looking to start a traditional academic career as a physicist, given the background in your answer, and try to give some perspective and practical advice. However, I want to emphasize that what I am not telling you is how to evaluate whether following this path is really the best thing for you when you consider all of the factors in your life and happiness; only you can make that decision.
The only way I know of to become employed full-time to write physics papers is to become a physics PhD student and get a PhD. The mainstream track at that point is to become a tenured professor (which itself requires several steps: typically 2-3 postdocs and then a 6-year term as an assistant professor before a tenure review). You should be aware that as a professor your job is not only to write papers; in fact most professors spend only a small fraction of their time doing research themselves, and much more time supervising their students and postdocs (who are in turn spending most of their time on research); writing grant proposals and reports; serving on academic committees; and teaching. In some fields you may be able to get a PhD and then do industrial research, rather than become a professor. My background is in theoretical astrophysics/cosmology/gravitational waves, so I am not super familiar with quantum foundations specifically, but you should research the realistic job options in the field you want to pursue, including salary, location, probability of obtaining a position, and whether these options appeal to you. You should also be aware that the pandemic has had an impact on academia in terms of available funding and positions, and the long-term impacts of this are unknown.
To get into a physics PhD program in the US [this may not apply in other countries], the typical route would be to get a physics undergraduate degree and obtain good grades, do research, get good scores on the GREs and Physics GREs, and have good letters of recommendation from professors familiar with your work, from research or courses. Of course you don't have all of that, and you don't need all of it, but you should know that your competition for limited places in PhD programs will have those things, and you will need to show that you have similar promise by being strong in other areas.
I think, as others have said, a good route would be to pursue a Master's degree. Typically you pay for a Master's degree, while a PhD program should cover your tuition and provide a stipend for you to do research (many students in the US go straight into the PhD program without a Master's degree). You should never pay tuition for a PhD program yourself, or feel you must have an additional job to support yourself (though you certainly can make money on the side if you want). So, with that in mind, on the one hand getting a Master's means you will need to pay for tuition. But, on the other hand, you are paying the University, rather than them paying you, so this tends to work in your favor in the admissions process. I also genuinely think that pursuing the Master's is worth it, since you will have a chance to study the subject, get good grades, get involved in research, and meet some professors who can write letters of recommendation for you.
I cannot emphasize enough how important this last point is. You need allies who is at the professor level who will write you strong letters. Your goal during your Master's should be to get to know your professors. Go to their office hours and ask them about their research and ask them questions about the course material. Ask to do research with them. Do well in their courses. Taking courses and doing research is very difficult and time consuming, and of course you won't make as much progress on research during the school year as you would if you were only focused on research, but carve out a substantial fraction of your time (10 hours / week) to do what you can. Spend the summers doing research.
It sounds like you know what field you want to pursue. In this case, you should focus your energy on the best possible preparation for that field. Do not get a degree in anything that is not directly related to quantum foundations. I actually don't know exactly what field you mean (you should clarify this for yourself), but I suspect quantum foundations is interdisciplinary enough that depending on exactly what you are interested in, you could find departments that cover this field in physics, computer science, chemistry, and maybe others. Look at departments before you apply for your Master's and look for places that have experts doing research in the aspects of quantum foundations that you are the most interested in. Write about these people in your application and be specific about how your interests overlap with theirs.
Other advice for applying to the Master's program: study for the GRE and physics GRE and get good scores. Reach out to former professors whose courses you did well in and ask if they would be willing to write a letter of recommendation. Spend some time crafting a personal statement which explains your drive and ambition and demonstrate to the committee reading your application your passion about physics by explaining self-learning activities you have undertaken. If at all possible, find someone -- ideally in the field you are interested in (a friend, former professor) -- to look over your materials and give you critical feedback before you send it.
Finally I would stop studying QFT, GR, and gauge theory. These topics are not relevant for quantum foundations, so you should drop them to focus on things that will help you more directly. Perhaps there are some areas where they overlap, but this would be a super niche research area and you need to triage and focus on learning the basics on what you are the most interested in. Based on the comments, it also sounds like you may have gaps in your knowledge of classical mechanics, quantum mechanics, and electromagnetism (although you were good with statistical mechanics). You must learn all of these subjects at the undergraduate level very well; this should be where you focus your effort in self-learning. These subjects are foundational and are assumed knowledge for any physicist. More to the point, you will need to demonstrate knowledge of these topics to succeed at the Master's level -- you will need to know this material to do well on the Physics GRE; to do well in the Master's level courses you will need a strong background so you aren't starting from scratch; when you start doing research, you will need background in at least some of these topics to be able to build and understand the topic you work on. You can't walk before you can crawl, and it makes no sense to read advanced topics in quantum foundations (or for that matter, GR, QFT, and gauge theory) without a strong understanding in these subjects ("strong" meaning "you can solve the homework and exam problems correctly and in a timely way which are given in a good undergraduate course.")
I'm adding a few recommendations that came up in the comments.
The OP clarified that their interests were in "many body localization" and "the mechanism that propagate quantum effects to macroscopic levels". This is not at all my area of expertise, but I recommend some keywords to search for these topics would be "condensed matter physics" and "Anderson localization". I think superconductivity is an area that is very active with a long history and very much related to coherent quantum effects with macroscopic amounts of matter.
For electromagnetism, I would actually advise against using Jackson as a textbook. It's famously painful. There are other books that are good, such as Zangwill.
It's not necessary to work through books written at the graduate level (those books are the foundations of a master's degree). Basically you want to be able to do the homework and exam problems of good undergrad courses.
Here's a link to an excellent MIT undergrad QM course as an example: ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/
David Tong has an excellent series of lecture notes on many topics in physics, which always start with an annotated bibliography: damtp.cam.ac.uk/user/tong/teaching.html I'd focus on the things that will come up on the GRE and Master's courses: Classical Dynamics, Quantum Mechanics, Electromagnetism, and Statistical Physics. – Andrew 13 hours ago
The Physics GRE isn't a breaking point especially since you can take it multiple times and send your best score. But it is important to do well on it and it also will give you a sense of what topics you should know. (You need to know things in a bit more depth than what's asked on the GRE, but I think to do well on the GRE you need to know things at more depth than they are asking, given the time constraints...) It is a bit of an annoying hurdle and isn't representative of physics research at all, but take it seriously.
In general (especially for self-study) it's best not to rely on one resource, but to have at least two from different perspectives so (a) if the explanation in one resource doesn't make sense you can get an alternative explanation, and (b) so you can increase the robustness of your knowledge by avoiding "overfitting" to one way of looking at things. In a lecture course, the lectures and the textbook already provide two different sources, but even then it's better to look at more.
