[LAIFscratch]

Get Skopenkov's 'Algebra via Problems' book to train olympiads: https://bookstore.ams.org/mcl-25 and his Geometry book https://bookstore.ams.org/cdn-1629826994644/mcl-26/ there's a combinatorics version coming out soon.

You want hard problems just above your level of understanding that when solved teach you dozens of different concepts all at once, that is what 'olympiad' style problems do. You won't be able to linearly go through all the recommended math texts here you will give up from boredom after the first n chapters because you aren't being forced to do it whereas a problem book it will annoy you that you can't solve something, and you'll want to solve it, in my experience. Failing that open up Concrete Math by Knuth and skip to the exercises, use the book text as your research material. At least it has written solutions if you give up trying to solve it. Repeat enough times and it eventually makes sense