It's tough going to bf, express, and make up formula - it's it hardest of all feeding options. Well done for juggling it all so far.
Have you tried a babymoon? Stay in bed with baby for a couple of days, with your top off and baby in just a nappy, maximising skin to skin. (If your bedroom is cold, wear a cardigan and snuggle up with baby on your chest under a blanket). Have drinks, snacks, books, Netflix, nappy bag, etc. on hand so you only have to get out of bed to go the loo. A partner to run around and cook you food is important.
If baby is awake, boob. If she falls asleep on the boob, leave her there so long as you're comfortable. No expressing at all. A couple of days of this does wonders for your supply, far better than expressing, particularly in the early weeks when your supply is still settling down. And it gives you a chance to rest, and baby a chance to practise bf (she's still learning too).
If she seems hungry even with the constant bf, ask your partner to run up a small (1-1.5 oz) bottle of formula. Then pop her back on the boob again. You can always give her another small bottle a little while later if she wants it, but give her every chance on the boob first and keep formula to small quantities each time. You wouldn't necessarily expect her to drink the same amount of formula supplement during a babymoon as during a typical day.
And after the babymoon, you could try to have her feed direct more often rather than expressing. Run the idea by a bf counsellor (NCT, LLL, etc.) and talk it over with your midwife / HV afterwards. I find that midwives are sometimes a little too keen on expressing as the solution to everything, when there are often alternatives available that may be better for you, and they're often happy to support them if you discuss it.
Try to let go of the idea of seeing how much she's drinking. I know it can seem reassuring, but it's not how bf works. If you establish bf, then your supply increases with her demand, even if she's getting formula supplements as well (so long as you don't ramp up formula quantities as well).
And congratulations! 