Assuming you aren’t breaking your journey by stopping for 1/2 days in DOH, by the earliest time food is served on your SIN flight you’ll have been travelling for 17 hours + and unless you do that kind of journey regularly my experience is you won’t know what (if anything) your body wants to eat until you get to that point. I’d be inclined not to pre-order for the second flight unless you’re a picky eater or something particularly appeals.