Hi suwoo
Have been racking my brains for anything else that you may be able to try. Apologies if this has been suggested already but have you tried observing the pattern of a feed to see if you can improve the milk-to-effort ratio for ds? Will try to explain as best I can, no idea if what I am about to say is common to all, most or many women, or just unique to me but thought it might be worth a shot.
When ds was small (probably up to around 20 weeks) it was very obvious during a feed when a let down was happening and when it wasn't. When I was having a let down ds would drink the same way an adult would drink a cold drink on a hot day. Gulp swallow, and the swallows were very pronounced and pretty much every mouthful. In between letdowns, it would be more like an adult drinking a hot drink, with swallows much less pronounced and less frequent.
After watching a few feeds after I had picked up on this, I realised that the pattern of my letdowns was very regular. The first one would come within 60-90 seconds, and last about 2 minutes. The next 2 would be similar with about 3-4 min gaps, so within 10 minutes or so there would have been 3. After that the spacings were longer and the duration of the let down shorter.
So I knew, for example, that once he had had 4 let downs, if he was a bit fussy there was no point trying to persuade him to feed any more because he would have to suck for at least 5 minutes not getting a lot, whereas if I switched breasts he would get more quicker.
I didn't need to watch closely the whole time because once I realised this I would just be dimly aware the whole time, and could either persuade him to hang on for the next let down or switch. (Also helped to watch a DVD during feeds - I could see how far the DVD had run and I would know at what point he was in the let-odwn process!)
I also found this useful when trying breast compression - sort of trying to 'milk' yourself into the baby's mouth when they are feeding. Worked best just at the end of a let down, and I would see him change from sipping to swallowing again for a few more sucks.
Not sure if the SNS would make this more difficult to spot - it may do but (for me certainly) the difference between let down and non-let down milk flow is so marked I think you would still spot it. When I express, I can get at least 20 times as much milk in a 'let down' minute as a non-let down minute - I know DS will be better at sucking during the non-let-down pahase but I still think the difference is about 10 fold.