Hugs to you and welcome to the boards
First off, I have no idea what PDA is or what that means for her growth, but could some of the slow growth be due to that? Have you been given any guidance for how she needs to be handled or food intake?
The way to get her to take bigger feeds is to not feed her before a certain point of time and keep tooth to let her adjust then lengthen it again. So, if she feeds at 7am and has been feeding 45 mins later you don't feed her before an hour and make sure that's the same for every feed. I would hold at that for a couple days and then lengthen to 1.5 hours for example. You may need to change to a faster teat so that she can drink quickly without getting tired sucking on the bottle. There will be some crying as you will need to distract her, so I guess my concern is that would the crying be too much for her medically?
Stretching to a longer routine means she has less bottles, but she should be taking more in a bottle so the overall milk volume is the same. I would expect a 5 month old to be on 4 day time bottles, maybe a dreamfeed and then 1 or two night feeds, so 5 -6 bottles in 24 hours. The volume is very baby dependant, but 7oz/ 210ml per feed is probably average, so over 1000ml in 24 hours.
I would stay off the rice to be honest as it's empty calories which fill the space in her tummy you need for the more calorie dense formula. If she's showing signs of food readiness you could look into Baby Led Weaning which may be a better option for her.
What do you think?
Laura