Oh gosh op I’m sorry. This can be so hard. My baby ended up developing a full blown bottle aversion when this started happening because I got so anxious I started pressuring without even realizing it (offering multiple times, getting up and walking around while feeding, distracting etc) and turns out my baby is really sensitive to pressure and it just made it way worse. The stopping at the same point every time (a common thing with bottle aversion, they eat jussst enough to get those hunger pangs away) makes me think it’s at least worth it for you to read up on bottle aversion to make sure that’s not it. I had to hire a (very wonderful consultant) to help us but once we followed the basic guidelines to ge through the aversion things got SO much better. So sharing to either prevent it from happening or in case it resonates:
https://www.babycareadvice.com/blogs/bottle-feeding/feeding-aversion
She has a book as well, and the consultant I hired was Lindsey right from this website and she is kind and helpful. I found out about this as a possibility from my LC. As you said pediatricians have no idea. It was my LC who identified it might be a bottle aversion (we had started a special formula for suspected allergy as well and we think he had a bad reaction to the taste that started all of this but honestly I’ll never know)