Mixing breastmilk with formula

My baby is 15 wks old and has a really bad reflux. She is currently on omeprazole and Infant gaviscon. Lately she has been refusing feeds, crying arching her back etc. She is combination fed 2-3 bottles of expressed milk a day and the rest formula - Hipp organic U.K (we live in the U.K)

The doctor is now keen to retry carobel (we tried that when she was 3 wks old but she became really constipated and upset) to see if that will improve her reflux issues. I have been reading a lot on oral aversion and have made the feeding time a playful enjoyable time but we have some good days and some bad!

If I offer breast milk she is more keen to drink it but at the end she pretty much has the same quantities for both Bm and Formula. I was wondering maybe she doesn’t like the taste? I was thinking to mix them but can you do that? I read stuff about overconcentration of nutrients etc?

Hipp is good for her as she doesn’t have any colic no vomits or constipation so I have been quite hesitant to try other brands?

Any suggestions?