My DD went through a similar pattern Lou222. She would feed like crazy at night from about 5pm and still wake 3-4 times in the night. So yes she seemed hungry. This has settled and she wakes far last most of the ime!
The mechanics of BF'ing - supply and demand - mean that as soon as you introduce any bottles your supply will be affected. The more you breastfeed the more you produce
At 6 mths your DS is probably going through a growth spurt - this can be height and development related not just weight gain! ( I remember being distraught at the amount DD had fed over a week and then only put on 2oz! but looking back her brain was developing and she was learning new things)
HV and paediatricians are usualy NOT Breastfeeding experts. I often felt I had to battle to continue exclusively breastfeeding but I would go through that battle 10 times over again if it meant I could continue breastfeeding again.
Breastfeeding is giving your DS the best start in life. HV and paeds should be congratulating you particulary as your DS was so prem. It's a real achievement and you should be proud of yourself and your DS for getting this far.
He is NOT failing to thrive. he is putting on weight. Don't worry about him not being chubby DD isn't at all but she is perfectly in proportion and more than capable of holding her own with her (much bigger) peers!
DD is still not on a centile line (has always been below the 0.4th) but she follows her own line and that is the main thing.
Do you not have a breastfeeding counsellor at your local hospital or a breastfeeding group you could go to for support? I was lucky in that I found a fab breastfeeding counsellor who was so supportive and without her I may have given up.
As I said before Breastmilk has more calories than formula so to swap is nonsensical.