Ah op. I ready this because my sister is 30wks pregnant with twins, she also has an almost 3yo, and I really want to be able to help her (if bloody covid let's me!) when they are here.
If you were my sister I would say :
Have a bath, have a cupa, sit.
Knock the expressing on the head. Unless you're getting a decent amount in a go, I don't think it's worth the time. It's hard enough to express in between feeds with 1 baby.
I would suggest trying again to top up with formula. There are different brands you can try, although they're all the same components. I know there's a lot of emotion around this subject. But honestly mixed feeding could be so practical for you with twins.
Everyone else is needs to become your slaves. They fetch. They carry. They feed you.
Extended family can do one. Unless they're practically useful and requested by you.
Don't underestimate the effects of sleep deprivation. It's hell on earth. Sleep whenever the opportunity presents itself.
As pp said you're in the trenches. Survival is good enough for now.