Hi Fimbly,
I am a year 13 and applied for competitive universities for maths.
Sounds like a pretty good list you made there. In your personal statement you only have space to write about 3-4 things. So I would not worry about quantity but quality. Would not set I need to do this task list of stuff for personal statement, but study what interests you. What they are looking for is how you critically engaged in those things, that suggest you have the skills to be a good mathematician. For example, I saw an interesting problem in Rightangle, school work, (or any source of maths questions) made me want to explore further/ generalise. I would say STEP prep is worth doing because they are a different breed of questions, and universities would recognise that.
If you have any questions, feel free to ask.
Junias