Expressing is not a true representation of what you actually produce. If baby was sucking he would be getting a lot more than 30-40ml and exactly what he needs. I've breastfed for years but was never able to express much at all. Why did you want to express? Were you worried that baby wasn't getting what he needed? How was his weight doing? Nappies?
I don’t think there’s a magic secret answer, mine are over producing which is also not ideal as uncomfy if he doesn’t want to eat as often and leaking like mad! It’s just our bodies adjusting and learning what amount they need, hopefully it’ll settle to the needed amount for us both soon! Ditto the above tho - babies will always get more out than a pump so just because that’s what you’re pumping it doesn’t mean your baby isn’t getting more (I also pump very inconsistently - the other day I only got 70ml and then next day I got 180ml pumping - our boobs are learning!)
Expressing and pumping is no where near as effective as baby! Diet and massaging boobs very gently for 10 min before a feed or pump helps too. Cluster feeding is just baby putting their milk order in! The more you get baby the latch even if it’s for few seconds at a time the more milk your body makes, as it’s the saliva from baby communicating with your body. Very clever system. Don’t worry about it too much, I know it’s stressful at times and really takes a mental toll but it will be over soon.