You don't need to do a second bachelors - you really need four or so courses. If you have the patience and dedication you can sit down with the textbooks and work through them on your own.
There's always more you might want to learn, but when people talk about these basics, it's really just being super focused in 4 or so classes, not a whole ivy league undergrad curriculum in math.
probability & stats, multivariable calculus, and linear algebra will take you a long way.
True for me. I knew all of these from my course work when I graduated with my CS degree in 1996. I haven't used them at all in my career, and so I'd be starting basically from scratch re-learning them.