It is true that AJX has very few Chinese pilots, in fact I can only think of one and he is Hong Kong Chinese, so similar to your situation. Obviously we'd never get mainland Chinese because they train with one airline in China and stay there for the duration. I can't imagine a Japanese airline being particularly warm to any mainland Chinese pilot but I do believe they would be much more inclined to look at someone like yourself who is Singaporean and, in my mind, a completely different kettle of fish. We do have a fair number of Korean pilots, both from Korea and American-Korean. I do appreciate the difference in attitude of the Japanese to those from Korea versus China but I do not think this exists with Singaporeans and I reckon you'd be given a fair chance.
If your BMI is above 28 then you'll need to get below that for the initial medical and, preferably down towards 25. Allegedly there is no limit in the JCAB regulations (although I've never actually looked) but that really doesn't matter, since the first medical is for AJX and they DO have a limit.
It will take you about 6 months to finish groundschool (plus all the associated courses), simulator training and get your type rating and ATPL. After your days off (normally 10) you'll then do 2 weeks of unnecessary airport 'training' in the classroom and then approximately 2 months of OJT/line training. It's a long slog and we all know it could be reduced to a more normal max of 3 months but it's their trainset (no matter how badly designed) so if you reckon you can let that stuff wash over you then submit an application. As I've said before; the worst they can do is say 'no'.