You won;t be the first mother to have a weep at a bf support group, that's for sure :)
What you are doing is exhausting. You are topping up and then guaging how much you ought to express and worrying when you don't manage it. Eeek! Babies often take a top up when they may not actually need it - they like to suck. Then they may take rather more than they need - it's not an indication that you need to express as much....though of course you are right that expressing when you miss a bf for whatever reason is protective of your supply. But what is even more protective is not topping up in the first place :)
She does not sound inefficient to me. Your definition of an efficient feed is way, way, way out....you say it has to be 40 mins inc a nappy change. Why? :) Who wrote that law? Some babies take more than this, some take less, some take more/less sometimes, and on other occasions do something different.
We are talking little human beings here, remember :). If you wanted to talk about an efficient washing machine, you might want your clothes washed and rinsed and spun in 60 minutes or whatever, and if your machine stuck on the spin, or didn't rinse properly, or took forever, then you'd wonder if something was wrong. But a baby, a person, is going to be more variable than this for all sorts of reasons, and that's something that goes with the territory of babycare, whether bottle or breast.
Energy in a baby is not something that comes and goes, in relation to 'good' feeds. A healthy thriving baby like yours will have different days and different needs and different moods, even. You 'tune into' her best when you go with her flow and stop assessing all the time. She is gaining weight - and that's down to your care and vigilance during the difficult time. Now you can relax a bit...yes?
You cannot feed her 'too often' - she best adjusts her intake to her appetite when you keep her close, feed her when she cues she would accept a feed...and if you think she still needs encouraging to feed that's ok but you don't need to sweat it, as far as I can tell from what you have written here. The urgency is over now :) Time to enjoy her.....