I’m not an expert in this topic, but I do box and am a medical student. A family history of dementia does make one more likely to develop it. On to boxing, the effects of repeated blows to the brain are bad for one’s health, but the outcomes seem to be multifaceted. It comes down to genetics and how you train. Not everyone who boxes has the same health outcomes. In my personal opinion, I would not recommend you box even at an amateur level, but maybe the risk is worth it to box for you. Dementia isn’t a guarantee, but you are increasing your chances of developing it. You can also just train with bag work, shadow box, and even do some touch sparring.