I think the idea of measuring milk is a huge bugbear for some BFing mamas, and once you start thinking about it, it can be really maddening! But, it really is not possible to measure output. What you are pumping is *not* a measure of what you are producing. Babies are much more efficient than pumps, and as they get bigger, they get more efficient. It's totally normal for her feeding to speed up, and it's totally normal for your breasts to feel much more 'normal' and much less of the huge feeling of fullness followed by real feeling of emptiness that you can get in the first six months. All that means is that you and she are working together really well to get her supply bang on where she needs it. If she wasn't happy, you would know all about it.
Without being alarmist, I want to add that if you do introduce formula at this stage, it will undermine your supply. It's a demand-led system, and if you reduce demand, by reducing the number of breast feeds, then supply will reduce too. That's something to bear in mind.
I feel perhaps you are not happy because you are nervous about your supply, but, honestly, it all sounds perfect. My two year old (yes!) is still regularly BFing and my breasts are not behaving anything like they did during that first six months. If you want to do something simple to boost your supply, you could try taking some fenugreek. It's great stuff.
Hugs for the nerves though