I did an undergrad CS degree after ten years of working as a professional software developer, and then went on to do OMSCS. My classes for probability and matrixes assumed no particular knowledge of them so I didn't regret not learning anything about them before. Learning proofs from scratch for discrete math was very difficult though.

I had to pick up the notation, the logical patterns, the prose style, as well as the new model of reasoning all at the same time. If I were doing it again I would definitely get at least very basic familiarity with how proofs work before starting.

Also just algebra and basic math skills. I was having to go back and relearn like middle school math, I didn't have confidence with basic things like multiply and divide fractions, exponents, logarithms, etc.

I was trying to learn trig on the side to keep up with calculus and it was a disaster, I failed calc 1 the first time. Given the constraints I had with work I'm not sure how I could have prepared better, it's just too big of a gap to easily fill on nights and weekends in a few months. But yeah learn algebra and trig before you do a CS degree.