My 3.5 week old usually feeds for 10 mins max! He is putting on weight so must just be an efficient feeder. Like your LO, if I try to get him to feed more he cries. I remember my DD suddenly went from 20 minute feeds to under 10 minutes at 8 weeks, and she never took long feeds again after that.
Based on around 8/9 feeds a day, the nurse has told me to give my LO about 70ml if he has a bottle for one of the feeds. Your LO is a bit older and probably bigger but I would guess 150ml is still a pretty big feed so maybe that is carrying her through so she doesn't need much at the 3:30 feed. My LO is still feeding 3 times most nights around 10, 1, 4 and doesn't take much at the 4am feed but then wants a feed again and up for the day around 6am... it's all still so changeable at the moment though.
I hear you with the frustration around naps... just when you think you've got it worked out the rules change! I definitely have my moments when I feel frustrated and overwhelmed and stressed out, but I am trying to remind myself to just go with it, as they are this little for such a short amount of time, so if it means a day of naps in the sling so be it.
