OP my goodness, this is so stressful.
I can't help regarding quantities or frequencies, but wanted to let you know about supplemental feeders. There's one called Lact-Aid and another that is more common. A lactation consultant will know.
Why they might help you is that you feed baby on your breast, so they're getting all the milk you're producing, but there's a tiny pipe/straw that you put in their mouth with the nipple that is attached to formula. I had to use this, in similarly stressful circumstances, when my son was newborn. It made a MASSIVE difference and enabled me to still produce all I could (breast surgery was cause for me).
Also, the lactation consultant, who was pretty "radical" about breast is best told me that you still get about 80% of the benefits of breast milk with only 20% of intake (the rest being formula). I never checked the figures and may have remembered them slightly inaccurately, but that was the gist of it. She also went from radical breast is best to "give him formula and don't stop until he does" before I met her when I explained his weight loss.
He's a super happy little boy with lots of friends and doing well at school. He has eczema and dairy intolerance, but I can't have dairy, neither can his paternal grandfather and his maternal grandmother had eczema too, so can't say what the cause was.
Your baby looks beautiful by the way.