Hiya
I would definitely persevere with feeding at "feed" times, what size of teat do you use for her? I found he was taking an hour to take 7oz a few weeks back and now he does great with the faster flow. Is she maybe getting frustrated with the bottle being slow?
I have been giving Calum baby rice and some light purees since he was 17 weeks, I just felt he needed more. I started Baby rice at night, it says a couple of teaspoons, however I always gave more than that. Also remember that formula has more calories than baby rice so it doesn't really fill her tum.
Certainly give baby rice for her age and weight, but look to also try and increase her formula. Calum can take anywhere between 10mins - 50mins to take a 8oz bottle

He only gets 4 a day and a watered formula at night if required.

He weighs 15lb 15oz, and is very long....
Another tip I give is to lay her on the floor if she starts to play around when eating or makes out she had had enough (but you know she hasn't!). I do this with Calum after he takes 4oz non-stop he plays and fusses at bit so I wind him and lie him on the floor for 5 - 10mins and come back to him and off we start again.