I am an electrical engineer and can say that the field is quite broad - I'm pretty sure mechanical is the same in that regard.
If there is a "core" to undergrad electrical engineering curriculum, it would be: analog and digital electronics, signals and system, electomagnetics, probability and random processes, and probably feedback-control systems (which I never took). Beyond this core you would have a whole host of specialties from information theory to quantum electronics to power systems to signal processing to semiconductor devices to control systems to computer engineering to electromagnetics (antennas, etc.) to ... I think you get the picture.
In my opinion, If there was one topic that could help a physicist think like an electrical engineer it would be signals and systems (assuming you have already taken electronics from the physics department). I have worked with a number of physicists we have hired at my company, and the lack of signals and systems (and probability theory - who knew one could get a physics degree without it?) is a noticeable gap. Standard books are Oppenheim and Willsky (1st edition is fine):
https://www.amazon.com/dp/0138097313/?tag=pfamazon01-20
and Lathi (1st edition is fine),
https://www.amazon.com/dp/0941413349/?tag=pfamazon01-20
The two books are fairly different but either would be fine - Oppenheim is probably the most widely used. MITs videos for the course the Oppenheim was written for can be found at
https://ocw.mit.edu/courses/electri...-science/6-003-signals-and-systems-fall-2011/
There you will also find homework assignments to work on. I would follow the MIT course, if I were you.
On the surface, signals and systems will look like a utilitarian course on continuous and discrete Fourier analysis, Laplace and z transforms, and simple ODEs; but beyond the math is the way it helps you think about time and frequency domains simultaneously, which is tremendously useful. Learning how to use block diagrams to represent both continuous and discrete time linear systems, including those with feedback, is crucial. This is not a tremendously hard topic, so you should be able to work through it at a reasonable pace.. This subject will have none of the rigor you are used to in theoretical math classes. But you should be used to this from your physics classes, where insisting on mathematical rigor would grind all progress to a halt in many cases. You can supplement (but not replace!) a typical book with ones that fill in the rigor, if you want. To fully do all of this rigorously requires real analysis (my math friends claim Lebesgue integration is really needed), distribution theory, and complex analysis.
Just my 2 cents!
Jason