Battling low supply... again

My first had “nipple preference” and refused BFing by day 5 after needing a bottle of formula on day 4. He lost 12% of his birth weight early on. I pumped after that and supplemented his formula feeds, though despite pumping for 30 mins every 2 hours and power pumping, my supply quickly decreased to only 5oz per day.

Today is day 15 for my second son and he is still slightly below birth weight. I haven’t needed to supplement formula, but we did just start (yesterday) supplementing with pumped milk. What I can’t understand is how he can nurse for 40-50 minutes every 1.5-2 hours (with me keeping him alert throughout and often hearing gulping and seeing milk around his mouth when he finishes) and then able to take 0.5 to 1 additional ounce after a feeding. Like how much is he even getting from me then after these LONG nursing sessions?

I’m hoping with increasing my pumping sessions (was doing 2x/day now doing 10 mins after each feed during the day), he will eventually not need extra in a bottle afterwards. When my husband goes back to work, I won’t be able to nurse that long and pump and bottle feed while also caring for my toddler. I also assume using a nipple shield is making the flow slower and more difficult to empty.... trying to wean off.

Ugh I hope this baby chunks up quick