Part of the issue (well I think anyway) which I mentioned in my original post, is her lack of feeding through the day, pretty much taking two or three minutes at a time,
Babies get much more efficient at breastfeeding as they get older. You just cannot place any assumption on the length if time she's feeding for.
It is totally reasonable (and most likely) that she's taking just as much in the 3 minute feeds now compared to 15 Minute feeds 2 months ago. In bottle feeding parents can physically see this happening- 6oz devoured in 3 minutes compared to 4oz taking 20 minutes 2 months ago, for example. It's just you can't physically see that happening when breastfeeding.
It's also irrelevant that night/evening feeds/take longer. That is usual too. It doesn't mean she's taking more, just being more leisurely about it. The longer deeds you mention do not mean more milk.
Similarly that baby is only taking one side - again means nothing, you are probably producing far, far more milk per side now than you were meaning one side is enough volume of milk.
So basically - 'tis the nature of the beast with breastfeeding - the never being certain if baby is having enough milk or not. You can't read anything into anything, just have to accept you can't know.
Getting her to feed more during the day would help as well
Having said all I've said above, there probably is a calorie deficit issue here too. That assumption isn't necessarily to do with her behaviour, more her age.
The run up to, and month or so after, the beginning of weaning is always difficult to manage in terms of getting enough calories into baby. It's not unusual for sleep to be affected at this age for that reason.
Rather than concerning yourself with how long baby feeds for, instead focus on how frequently baby feeds - so feed more often. You can't make baby's stomach volume physically increase so if baby is full after a feed then you can't change that. But feeding more often will get more calories in.
Your not there yet and being a FTM you won't know, but once you start weaning then feeding baby goes back to how it felt with a newborn. It can genuinely be like baby us spending all of their time either feeding or sleeping - literally for the whole day.
So before you start weaning to solids, you could start milk feeds at the frequency you will be feeding post-weaning. Bear in mind that from 6m to 9m baby will maintain breastfeeding levels you now have, maybe increase. Plus solids on top. A typical day for my youngest at 6-7m looked like this:
7am milk feed
8-8.30am breakfast porridge (spoonfed)
8.30-9am fruit or toast fingerfood
9am milk feed
9am - 11am nap
11am milk feed
11.30-12.30pm puree savoury spoonfeed. Then lunchtime finger food. Then yoghurt spoonfed. Then fruit finger food
1pm milk feed
1-3pm nap
3pm milk feed
4.30pm milk feed
4.30-5pm power nap
5.30-6.30pm puree savoury spoonfeed. Then finger food whatever the family meal is. Then yoghurt spoofed. Then fruit finger food.
7.30pm milk feed then bed
As you can see - that is a lot of feeding!!
With that in mind, compare to how much/often you give milk feeds now. Granted my DD had a big appetite, but it's not unreasonable to say baby may well be wanting full feeds every 2h. Possibly even more frequently than that.
Its an incorrect assumption that frequent feeds mean baby is taking lessmilk per feed. If you were bottle feeding it would not be unreasonable for some babies to be having 7oz (Which is a very big feed) every 2h. It's just when bf you can't put a number on it.