For people who more experienced than me:
I also use the Cambridge textbook and am in the same camp in that I only get around half the enrichment questions right. Is it necessary to be able to get most of them right, or just an understanding of the concepts enough?
Enrichment shouldn't be an indicator on how well you'll do on the topic itself. Sure, if you're doing good in enrichment it shows you are more than capable in learning and understanding the concepts of the topic and possibly to a further extent, thereby meaning you might be more adept in doing harder questions, but does not fully correlate to how well you will do in exam condition.
I personally did every enrichment question possible just because, to kill time or as an excuse to study english less or bc they seemed cool. But I would attribute my exam performance mostly to being able to solve routine questions extremely fast and having enough time to tackle any challenging questions. You'll find the exams are designed in a way such that most problems are basic and require minimal thinking (focussing on "do you know your content") and the rest few are supposed to differentiate people who really studied the topic and those who just studied the bare minimum.
Focus on getting used to the exam condition, try to get in to a practice of doing problems in a short amount of time ; timing yourself will somewhat emulate the pressure you will feel in exams making you more better in that setting.In exams, I'd always find that its the nerves/pressure that would get to me at times and would lead me to make mistakes I'd never do so thats an aspect of exams you should be ready to tackle with. Silly mistakes is a big differentiator for many people so reduce the errors in your working out.
TLDR; Don't use enrichment questions as a benchmark for your ability, moreso your persistence and improving your problem solving skills. Do them if you want (highly suggest) but ensure that in weeks leading up to the exams you are more pragmatical in how you study. Saying this from experience.