sinmey, Mirena supposed to give light menses flow, some even no flow, whereas normal copper IUD will give more menses flow & cramps. So I guess Mirena may be more suitable for you, but it's more expensive.
SN, I'm still pumping every 2 hours @ 30mins in the day (unless I go out). At night, if I can wake up every 2 hours, I try to pump 20min. If I oversleep, then I try to pump 30min. Cos very tiring & cannot tahan. But if I pump shorter / less frequently, the amount also decreases by 10-20ml. Your boy is drinking quite a lot. No wonder you have a hard time catching up.