If you are willing to butcher her, that is probably the best choice.
If you are not willing to do that, then probably continue with limiting her feed, perhaps an even smaller amount than you have been feeding. Make sure she has access to plenty water (I can't tell if she has trouble getting to it or not.) Make sure she stays fairly clean (so she doesn't get sores from laying in her own waste). In the video, it looks like she is on clean grass: good!
I would think the total amount of protein matters more than the percent protein.
1/2 cup of food at 20% protein will have less total protein than 3/4 cup of food at 18% protein.
If you feed a lower protein food, you are most likely giving the same amount of energy (calories) with less protein. I suspect that feeding less food is more helpful than changing to a slightly lower level of protein.
Unfortunately, I agree with this.
I see that
@Weeg has some other suggestions-- I don't know enough to tell whether they will help, but I don't see any harm in trying them.