Well, if it’s fanfiction w/o a beta reader, one could get away with mixing up the two terms “blonde” and “blond” every several sentences
A quick check of the thesaurus gives “fair”, “light”, “bleached”, “flaxen”, “sandy-haired”, or “yellow-haired” as synonyms as possible alternatives. But, yeah, that’d be my approach, check the thesaurus for non-gender synonyms to substitute during the descriptions. (I’m writing a fic where I’ve got a similar issue, though not in gender.)