Don't think of your breasts as 'full' or 'empty'. They're not bottles. You can't cry more tears than you can produce. You don't 'run out of sweat'. It's the same principle. The flow slows, which is why babies get frustrated, but actually the fat content is higher when your breasts are less full, as the milk is being pulled from deep inside the breast and bringing more fat down with it. The beginning of a feed, when your breasts feel fuller, is actually the watery, thirst quenching milk, that won't necessarily keep them full for long. Your breasts shouldn't feel full ideally - frequent feeds and efficient milk production mean less engorgement.
The reduction in nappies is slightly concerning, along with a reduction in poo. I would personally try and encourage as many feeds as possible. Try staying on one side per hour and offering every time you hear even the slightest squeak. Don't let her go more than 3-4 hours without a feed, including at night. A feed (or several) between 1 and 4am will do wonders for your supply. Evening fussiness is normal - check it's not wind, but basically let her feed as she wishes. It's all to do with building supply. Does she have a dummy? If so, I'd ditch that for now while you're getting sorted.
Keep an eye on those nappies, and if things don't pick up in a couple of days, consider calling professional support - ideally an ibclc, or one of the national breastfeeding advice lines x