Came here to recommend this. I completed my bachelors in mathematics back in 2003. However, recently I am thinking of going back to school to study graduate level mathematics or higher, and possibly do a PhD in a related field. My tentative plan over the next couple of years is as follows

1) Art of Problem Solving - all of the books to review mathematics at middle/high school level 2) Roughly follow the plan mentioned in https://www.susanrigetti.com/math to redo an undergraduate degree 3) Also complete the books here - https://foundations-of-applied-mathematics.github.io/ 4) Move on to more advanced graduate level mathematics