You can defo improve
Iirc I got distinction in year 5 and high distinction in year 6 for math and that hasn’t had much of an effect on me
Also for writing and reading I usually got credit and for science I mayve gotten participation once
I haven’t done ICAS in a while but I don’t recall there being a d. If by d you mean distinction you’re fine. If by d you mean a literal d mark, you’re fine.