I got to a similar scenario to you OP. It took a fair while to get the baby back up to it's birthweight.
I thought I would share my story.
What helped me was I got to go to a group with prem babies who were breastfeeding. The women and some were ex nurses who ran that group were non judgemental and fantastic.
I realised I needed to feed feed feed. Every 3 hours was too much of a gap.
So my husband had to support that. The mixed feeding was affecting my supply. My baby didn't cry, I think this might have been because he was under weight and just too tired to even whimper or cry. I also would feed when he got better because I could read his cues, so as soon as he started moving that little mouth I would try feed, so I would not wait for the baby to cry.
I had to try feed I think in clusters and then the baby would go a bit long for sleep - but most of the day it seemed like feeding feeding feeding.
So many days it seemed I was in bed, and just feeding until the weight got up.
I was also taught to tickle feet and stroke the inside of the baby's hand to get the baby to latch. The baby also had to get used to feeding more frequently. I never ever could express, not a drop.
But the baby and breastfeeding it got better. I stopped going to get the baby weighed as at first that causes me some anxiety.
By the way I went back to work FULL TIME at 7.5 months and still could not express but this was when the baby was a bit older obviously, the baby wouldn't take formula at nursery either, but when past six months would have yoghurt, cheese food but never milk and then I would cluster feed in the evening, I never expressed. I never thought I could do it without formula either.
My second baby I never expressed either.
I found that when I embraced cluster feeding from late afternoon to through out the night it kind of worked. But that is just my experience.
I never ever thought it at the time but I miss those cluster feeds.
Good luck whatever you decide and hope you get the support you need.
Also congratulations on your baby.