Sounds like you have high lipase in your breast milk! I read about it and seen a lot of it online. I saw mummies say to drop some vanilla extract. No need so many drops, just 2-4 depending how much your baby is able to tolerate the BM. If you’re worried, maybe can talk to a paediatrician regarding high lipase in BM.
I feel u, mine also had the fishy smell back then.. can smell when warming the milk.... so eventually I stop freezing and just feed baby fresh BM if possible. the frozen precious one can only milk bath.. I was a just enough supply so in the end I supplement with a bit of FM.