Any thoughts or ideas would be really appreciated.
Edit: I'm also really scared of "getting found out". It greatly adds to my feeling of imposter syndrome.
Also I really want something that I can't just work through, a structured path so I can see my progress and not constantly feel like I should be studying some other area all the time.
Linear algebra is important, as is combinatorics. Look at the required classes for a CS (or math) major, subtract the ones required for a physics major and I’d say that is your curriculum.
My perhaps old-fashioned view is that taking some kind of course -- online or otherwise -- that involves a prepared curriculum, work assignments, and examinations is probably the way to go.
Piecemeal learning may be effective under some circumstances, but for a large field that involves inter-related concepts, my guess is that it's hard to build a solid understanding without having stepped through a carefully-designed set of lessons that highlight and anticipate the connections in advance.
I'd also expect that finding the time to take that kind of course and commit sufficient energy to it may be a challenge. Could you ask your employer to see whether they'd be supportive of it? (perhaps with career and role development in mind?)
Another potential factor to consider: having a supportive social group during education can be very effective, I think. There's no magic solution for that; it'll involve a bit of outreach and trying to find those people to learn with.
That doesn't provide any neat answers or links for you, I'm sorry about that; but I'm curious to follow this thread and the responses, and to let you know that you're not alone. Good luck!