What I don't understand is why supplementing started at 1 month old, as baby's weight gain seemed to have become normal at that point?
When you started supplementing, did you/do you always pump each time you supplement (approximately) to 'make up' for that supplemental feeding?
Also- how often does your baby nurse? When I hear a mom whose baby is not gaining well say "she's a good sleeper' that makes me suspect baby is not being brought to the breast with enough frequency. This could be due to baby not cueing enough, but then it’s important for mom to not wait for cues and go ahead and offer/gently encourage more nursing. Even short nursing sessions help, its the frequency of the milk removal that is most important. If baby absolutely will not nurse any more frequently, then pumping with a very good pump frequently is the next best thing, but not nearly as effective as a baby in most cases.
Milk supply is dependant on demand. Whenever a mom supplements and/or baby does not nurse often enough, she runs the risk of diminishing milk production. The way to build it back up is with frequent effective nursing and pumping as needed. Plus galactagogues if you want.
I strongly suggest the book Making More Milk. Kellymom.com also has excellent info on increasing milk production. Based on your baby’s history, it is very likely you did not have some pre-existing condition harming milk production, as it appears the initial issue was the tongue tie. So, like the vast majority of mothers, in all likelihood your body is perfectly capable of making enough milk, you have just been taken off track by first the tongue tie and then the supplementing.
Also if bottle flow preference is a problem, you could consider using a lactation aid to supplement (helpful in many ways.) If that is not for you, try paced bottle feeding.