"An Introduction to Statistical Learning" was written by two of the same authors, and is explicitly meant to be a lower-level introduction to the same ideas: https://statlearning.com/ISLR%20Seventh%20Printing.pdf

Rather than a lower level intro, I'd recommend you bone up on linear algebra and multidimensional/vector calculus. The ideas in this book are not especially difficult to get but the math is right up front all the way through. If you can't actually read the equations and visualize what is being discussed, it will be quite difficult to ever make real progress with this book.

Try "Computer Age Statistical Inference" by Efron and Hastie.

https://web.stanford.edu/~hastie/CASI_files/PDF/casi.pdf