How much/often is baby sleeping in the daytime?
I don't know if you realise but babies who have no reflux of any sort also do these sorts of things.
Settling for an initial longer chunk of time then finding it harder to settle is normal, a baby without reflux would do this. It's often a sign of not enough sleep over 24/48 hours causing over tiredness making it hard to settle.
Many babies make grunting, sniffling and generally odd noises.
Pulling legs up does not necessarily mean anything. Remember your baby has next to no muscle control, the leg movements are largely random in the newborn, or just mean "distress" - which could be tired, hungry, feeling exposed, need comfort.
All that said, while it certainly could be reflux, I wonder if there is a possibility that you are adding to your stress by medicising normal baby behaviour in the hope of "fixing things". The fact that baby was previously sleeping well and that not having gaviscon made no difference - I'd sack it off and focus on breastfeeding and sleeping if it was me.
Have you tried a swaddle?
Have you tried a dummy?
Try to not let baby stay awake for any longer than 45 minutes at a time in the daytime.
Do you think baby is feeding well? Your HV will be able to point you in the direction of breastfeeding support, if needed. But don't be afraid of formula. If you are struggling with PND and would like to take the pressure off yourself, it is ok to move to mix feeding or formula feeding.