I took stats in undergrad, but it was a very rudimentary "push x sequence of buttons on your calculator in y situation" ordeal and left me with less applicable knowledge than I'd like.
Since sometime after graduation, I've taken up serious study of higher-level maths as a hobby. I think it will be most useful in my career if I also have a strong grasp on probability and statistics.
[edit] The suggestions so far have been great, but it occured to me to add that I'm working through Spivak and Apostol right now with Dedekind's essays on the side. Hopefully that gives an idea of the tone/rigor I'm after. Answers to problems is also ideal.
There are accompanying lectures, problems, and accompanying solutions on its website https://projects.iq.harvard.edu/stat110/home