What’s some hard university maths content?

The thing is most of university maths is completely separate in style and substance to school maths, so it's probably not as useful an experience as you think from a study skills perspective. Being able to prove idk, that two rings are isomorphic is probably not really useful to you at this point.

The challenge in university level maths at the start at least is less "study skills" and more just the fact it is so different from the kinds of maths you've encountered before. What would be useful is looking at some degree level maths topics not to "challenge yourself" but just to understand the nature of what maths at degree level is like (whether or not it's conceptually or computationally difficult is not the point, the point is to see how different it is.

I'd suggest looking at some introductory real analysis/(abstract) linear algebra/abstract algebra or maybe number theory or general first year "foundations" topics (mostly set theory and logic plus some various assorted topics around numbers and functions) and seeing how it's presented (theorem, proof, lemma, theorem, proof, lemma...) and also how you would be expected to "do" maths at university (i.e.largely proof oriented).

Some suggestions:

This reads very arrogant, so I’ll apologise in advance. Rest assured, I wouldn’t say anything like this in real life. Basically, I’ve managed to get through high school maths without studying at all. I sat Advanced Higher maths in S4 (2 years early) and got an A. This year (S5/Y12), I’m sitting 2 Advanced Highers and 7 highers and I’m coping pretty well with the workload. Hence, I want to be humbled. I see often that people in university will struggle due to not having developed study skills. Hence, I’m looking for some uni-level maths which I’ll find really difficult in order to hopefully develop these study skills. The problem is that subconsciously, I still think I might breeze through university due to the fact that people say online that the exam workload I’m doing is really difficult yet I’m coping with that. This attitude may not serve me well in the future.

Not to try and insult you, but the chance of you breezing through university maths (if your gonna do maths) is slim to none like literally none. That **** will be miles harder than any secondary school education.

That is the likely scenario. I guess that the problem is that, since I’m only used to high school education, that there’s nothing I can tangibly compare it to at the moment.