A few things I can say, and am happy to have replies which expand or limit or clarify what I've said:

Generally speaking raising brain creatine phosphate levels comes secondary to raising muscle phosphocreatine. It has been suggested that daily 3-5g dose of creatine monohydrate will overtime, and with vigorous exercise upregulate creatine phosphate pathways in muscle, but is unlikely to raise phophocreatine levels and/or pathways inside the blood brain barrier. There appears to be no adverse effects to moderate supplementation on the kidneys in individuals without kidney (eGFR) disease. Hmm, what might one consider a moderate dose? 3-5g/d would be considered moderate amongst experts and informed individuals. Some experts might say taking 10-20g / d, in divided doses is currently considered moderate, and may be sufficient to raise phosphocreatine levels in the brain. Creatine tends to be low amongst vegans. Chicken eggs are relatively high in creatine.