Ru,
Yes, there are little thiings we can do that ease the rejection and increase milk intake but usually will not totally eliminate.
1. Teat issue: enlarge it a little and see if she can drink faster. If she can swallow comfortably, enlarge in further and check teat flow (right flow should be that milk can drip out on its own nicely. Not gush out but drip out) every now and then. I find that sterilisation causes the hole to shrink over time on its own. So good to check flow regularly.
Also, replace teat regularly even if using silicon teat. Silicon teat hardens quite a bit with time and now I change every 1.5 to 2 months. Everytime I change, she drinks faster (as its softer).
2. Teething. Really come teething time, milkstrike can become very serious. Can try to feed with spoon. Pigeon has this bottle that has a spoon attached to the screw cap. Can use that to feed if really really don't want to suck. But risk of course is that bb may become used to it and reject teat although. Now, I feed some with teat and once she really don't want to suck anymore (say left 30ml), I will feed the remaining milk with spoon. Its not good but no choice, desperate to increase her milk intake.
3. Before 4mths is too early to start. Depending on her weight gain. From what most pds tell moms, if still got ok weight gain, recommended to weight until quite close to 6 months. However, if weight gain is not ideal at all, I read some pd oked bbs to start once past 4 months but only if weight gain is worrying. If weight gain ok, but not so ideal, maybe close to 5 months. So perhaps u can monitor, once u think her weight gain is stalling due to low milk intake and she is definitely past 4 months to 5 months, can start to train her with spoon liao. Use a proper weaning spoon and start with rice cereal, one teaspoon first. Am quite glad I trained her fr 5months plus as she took a while to reach the 4 tbsp per meal now. And if don't want to drink milk, at least can feed solid (add milk inside to increase milk intake).
Of course, reduce distractions during feeding. But on the other hand, there are bb whom once u on TV, they start to suck well. So it might be necessary to experiment. I have heard of desperate parents who let their bbs watch their favourite cartoons during milk time. It’s a function of personal belief and how desperate one is. Alto the general rule of thumb is to reduce distractions and just focus on feeding.
4. The next time u see ur pd ur jab, ask for the sea salt spray and keep one handy. Also get a Pigeon nose sucker. If suspect there is mucus, can use spray. Pd said spray can use max 3 times a day, not a drug, so safe to use when needed. After sprayed for about 1/2 hr, suck out whatever mucus with nose sucker.
600ml plus is not too bad. Your gal is still doing fine