Hi Aitch,
Sympathy with your LO. My DD was slow to gain weight, though not premature, indeed HVs still get very antsy about her 4th centileness. But she is a very healthy, happy, smiley baby so I'm beginning to think (at 9 months today)I just don't care anymore. (Proof of general well-being on my profile!)
But it's not a nice feeling at all and I sympathise.
She is gaining weight, so well done! We did top up but AFTER a feed. To give her a bottle before sounds like craziness to me, unless you want to bottle feed? I think, on this trust your instincts.
When Dh and I were in the middle of all that craziness, he did a little experiment as my bf was being seriously undermined by HV. DD was weighed every four days, so we did formula top-ups for four days and then breast milk for the next four days and she put on double with the breastmilk which made DH and I relax, stop agonising over strategies etc and do what we wanted (which was to give formula at times BTW). Maybe a little experiment along those lines of your own?
I feel for you because I found it so rotten. But the 'proof'we had of the way we wanted to go made us feel stronger.