Hello & welcome, I do apologize that my colleagues could not answer your question while I was away. Still, I am Dr. B, a licensedveterinarian and I would like to help.
Based on your history with Peck, you have been doing a very good job. Some birds will not be as immune robust as others ad we can see those "sensitive souls" struggle in the face of such infection. Now if she is doing so, I would note that we'd have to consider whether this is what she has or if this is only what she has. So, if she is really struggling, then it may be worth having her tested by your local vet to confirm coryza and also make sure there is no other opportunistic agent that is lurking (and perhaps not sensitive to the treatment you have used). Otherwise, just to note other antibiotics options that this bacteria is sensitive to Erythromycin, Oxytetracycline, Streptomycin, Dihydrostreptomycin, and the sulphonamides. So, you could consider trying an alternative if she is not making progress or modifying treatment based on any test/culture you have done.
Finally, your feeding sounds perfect there but I would just note that if you are having any difficulties with this, I would note that you can tempt her with favorite foods, water laden fruits/veggies (ie oranges, melons, cucumber, etc) and other foods (ie oatmeal, cous cous, brown rice, etc). And if we need to sneak more nutrition in, then we can use Nutrical paste as well for Peck.
Overall, your treatment has been good here and it just sounds like Peck has been hit a bit harder then most. Therefore, we'd need to keep up treatment as you are, consider the above, +/- think about an alternative antibiotic to make sure we are adequately addressing this for her.
Justin case you do need an avian vet and do not have one already, you can checkwhere you can find one at near you at:
Lafeber database(http://lafeber.com/pet-birds/find-an-avian-vet/) or Birdsnway(http://www.birdsnways.com/birds/vets.htm)
All the best,