* Of course you can transition to physics (or other physical science or engineering major). The question is. “How much time, effort, and money are you willing to expend on the transition?”

* A more fundamental question, though, is, “Should you transition?” Right now, the answer appears to me to be, “Probably not.” We’ve had other posters that have written along the lines of, “I’m a major in field X. Over the summer, I started on a lab project with a professor in field Y. I’m continuing with the project this semester, and I now realize I’m more passionate about field Y than field X. I’m considering switching majors. Should I?” Now those posters are on a strong footing and well poised to transition from field X to field Y.

But you wrote above:

“... However, I increasingly feel that many of the questions I am most drawn to are not central to modern ML; I find myself wanting work that is grounded in physical systems, requires lots of mathematics, and is more directly connected to nature.

Because of this, I have been thinking seriously about whether I should try to move toward physics, applied physics, quantum science, biophysics or some adjacent area such as ECE, applied math, or chemical physics. ...”

and

“... 1. How realistic is it to transition from a CS/math/ML undergraduate background into physics or applied physics at this stage? Would it be better to aim for physics PhD programs, or for adjacent programs like ECE, applied physics, quantum science/engineering, materials science, applied math, or chemical physics?”

You’re all over the map. All this tells me is that, other than a vague inclination towards a less abstract, more tangible field, you really haven’t identified a specific field, such as physics, that you want to transition to.

* Given that you will be entering your senior year, and given that your current major has reasonable job opportunities, your number one priority should be to ensure that you complete the requirements for your current degree. Even if you choose not to stay in the field, it would serve as a solid basis for a Plan B.

* In the US, the chances of finding a funded Master’s program is slim. In particular, I wouldn’t even consider a terminal Master’s program in physics. As discussed often in previous threads: (a) In the US, many of the top physics programs don’t even let you apply for a terminal Master’s program. At these schools, you get a terminal Master’s only if you apply for and are accepted into a PhD program; you complete the course requirements for a Master’s; and you fail to complete the requirements for a PhD (for whatever reason). (b) A Master’s degree in physics affords you little advantage over a Bachelor’s in career opportunities. You really need a PhD. This is not the case for other majors (such as computer science, computer engineering, electrical engineering, materials science and engineering, ...) in which a Master’s gives you advantages in the job market. In summary, transitioning to physics is the most difficult choice since it requires a strong grounding in foundational courses and a PhD. Unless you firmly establish that you have a calling for physics, there are easier choices to transition to.

* You also wrote above:

“I took an advanced mechanics course following Morin, and I saw some E&M and thermodynamics in high school, but I was much more focused on bio/chem at the time. I have notably not taken the full undergraduate physics core.

I've spoken to some faculty who suggested I study some quantum over the summer and try taking an advanced quantum course in the fall.”

If you’re seriously interested in physics, you should concentrate on coming up to speed in basic and intermediate E&M, thermodynamics and statistical mechanics, and basic and intermediate quantum mechanics. Studying basic quantum mechanics on your own over the summer and signing up for intermediate quantum mechanics in the fall strikes me as a recipe for disappointment and angst, things you don’t want in your senior year.

* One option is to tack on a fifth year to find your calling. But a better option is along the lines of what @Muu9 suggested. See if (upon graduation) you can land a job as a research assistant in a university lab. Some universities will allow their employees to take courses for free; check into that. Otherwise, try to land a job in industry with your credentials, skills, and experiences that will expose you to more applied fields. Then see what grabs you and transition to that field. E.g., I was an industry mentor to a physics undergrad. In her senior year, she really wasn’t strongly motivated to apply to a PhD program. So I advised her to apply as a lab assistant in industry, with the caveat that she probably wouldn’t be happy there long term. So she got a job as a lab assistant in a company designing and manufacturing accelerators for medical applications. Sure enough, within a year, she wanted out. But her job had allowed her to interact with medical physicists and she got intrigued with medical physics. She ended up going for her PhD in medical physics. She's completed her residency and has launched her career as a medical physicist.