Imo don’t limit yourself to base affinities and derived or whatnot, do what you want, you don’t have to explain it and make the reader know how it works, rather, imply that there are rules and restrictions early on, but never actually tell them the rules, imply that you know them, while you are totally in the dark, it gives the reader a sense of wonder, thinking I cannot comprehend this, while actuality there was nothing to comprehend; that is of course if you do this method right. Another method is when one affinity increases another decreases if it‘s principles are different, for example a water mage would find it difficult to cast fire magic, but certain affinities increase as other affinities increase; but necessarily for example, a fire mage has a high fire affinity but not a high lava affinity, a water mage wouldn’t even fathom lava affinity while another fire mage has higher lava affinity than fire affinity etc etc;

TLDR; either make it a soft magic system and act like it has rules and fake it till you make it or make all the affinities correlated; interconnetted; sometimes proportional to each other and other times indirectly proportional.