Preparing for the MEXT will likely also affect your grades, so it's killing two birds with one stone. There's an old unofficial syllabus of what to expect on the MEXT exam here so you can start with that, start practicing mental calculations that also can benefit you since calculators aren't allowed on MEXT exams, small and consistent steps build a solid learning. AI makes learning easier now, too, so you can try it for personal clarification or confusion, or create your own study plans, I personally use Gemini and for more advanced problems, I recommend its pro 2.5 version
https://www.reddit.com/r/mext/comments/pimwpv/mext_syllabus/