Hold your nerve, kiddo.
babycarrot gained a couple of ounces a week despite every conceiveable effort. I didn't top up (apart from cheating a couple of weighings).
In her first 6 months she only gained 3lb. I was demented with worry. In the end I decided that, as far as bf went, I knew more than the health professionals, and ultimately, I didn't accept that their advice was correct. I bought my own scales and started weighing her myself, and got out of the system as soon as I could. They considered her "Failure to thrive". I considered this a mis-diagnosis. She is the healthiest, happiest baby I know, and still breastfeeds several times day and night at 14 months old. Her weight really took off at (baby-led) weaning, and then stabilised again, and she's back to gaining a couple of ounces a week.
I just think some babies gain like this, and yet nothing is wrong. Tiktok once told me dd was like a bush baby of the Kalahari (or summat), designed to feed even as much as 20 times a day or more. This really helped me. If she wanted more milk she could have it, as often as she wanted.
Gosh, I know how awfully hard this is for you though. And of course you need to check that there aren't any problems that need fixing.
But you may have to consider the fact that you already know more about the best way to feed dd2 than the people who are supposed to be helping you.
Hang in there, Aitch.