If the new Ninja 250’s seat height is to high for you, look at the older models (2007 and older). IIRC, the seat height is a little lower for those models, and there is a ton of foam in the seat to shave down. Shaving the seat should get you to were you want to be. I’m tad shorter than you and I can tip toe the bike without the seat shaved. It would be better to shave the seat down to get the height you want rather than lowering the bike.