What are the signs of a low supply?

My little man is a little over two weeks old. I am currently breastfeeding, but I feel like I might have a supply problem OR I am doing this whole breastfeeding thing wrong. I think I am pretty attentive to hunger and full cues — open/closed fists, rooting, “searching” for a boob, type of cry, etc. My feeding sessions are anywhere between 45 minutes to two hours (with alternating breasts). It seems pretty excessive. I read some forums on What To Expect last night and a majority of moms have much, much shorter feeding sessions. During the day, my little man constantly eats at least every two hours. At night, he feeds for about an hour or so. He does have an upper lip tie, lower lip tie, and a second degree tongue tie. His latches are OK, a bit worse on the right side. At birth he weighed 7lbs 15oz. Weight went down to 7lbs 9oz at his three day check up. At a week and a half, he was 8lbs 3oz and our pediatrician said that’s good weight gain and wasn’t concerned.

So, what are the signs of a low supply? I always feel so “deflated” by the afternoon 😔 Should I consider combo feeding to ensure fullness and monitor intake amount? My lactation consultants were very adamant about only breastfeeding unless there was a medical reason to incorporate formula. I worry about digestive upset with combo feeding. Does any of this sound like an issue or am I just overthinking it? Growth spurt, maybe?

Thank you!!