Hi! My baby has been drinking NAN Optipro all the way, so not sure if any formula differences
But just wanted to share that the formula guidelines are really just for reference and don't need to follow exactly! My baby also had bad reflux when younger and was also very easily fulfilled, always drinking way less than what formula can recommended. But he is above 75 percentile for weight all the way. 😅 looking out for baby's cues is more important :)
Feeding less and more frequently really helps with the acid reflux so imo you're already on the right track and just have to wait for baby to grow up more before he can take a larger amount and longer time in between bottles.
Now my baby is almost one years old, eats three solid meals a day and max also only drink 150ml during milk feed.
All the best!