Have been pointed here by Floria - your description of your baby right now is just how I was last year.
From about 4 weeks, M would fuss at the breast, arch her back, scream - the works.
Firstly, let me reassure you - it does pass - this I can promise you - my bf relationship with dd took quite a while to be fulfilling - now it's the best it's ever been.
Secondly, M was excl. bf until 6 months - despite the reflux meds (I used Gaviscon quite successfully), and, in fact still bf now (at 14 months)
You don't have to give the gaviscon in a bottle: mix it up with a little ebm, and spoon it / syringe it into her mouth. I'd suggest you give her doses at the times of day when she's worst to start with, and work back from that.
Also, to reassure you, if your dd was on ff, and Gaviscon, she would be more constipated than she is now - you are doing the right thing entirely.
I'd suggest you look at kellymom - and that you get in touch with your local branch of la leche league - it may well be that you have a fast let-down reflex, which can mimic reflux (so I understand it) ... you would probably benefit from tiktok's help too ...
Good luck - thinking of you!