Did you get a culture?
Based on the prescription and length I would suggest another vet one that specializes in reptiles.
Oral antibiotics are not adequate for reptiles and prescribing this was just a waste of time and money.
The first step is to get a culture to established with certainty which type of RI you are dealing with and which anti-biotic will be a better fit, treatment will injection lasting at least a month.
Right now feeding this animal is far from being the priority given it's size, animal that are sick and under the stress of treatment rarely eat.
Do not soak your snakes on a daily this with a combination of bad husbandry and stress will lead to more issue such as RI, provide a water dish that cannot be tipped.
As for the rest a little food for the thoughts. You could not afford the initial treatment at the time but went on with buying more animals (Knowing this one was sick) while having no quarantine procedure in place............. that is a recipe for disaster, hopefully the RI is just that a RI and no something worse that could claim your collection.
You are young and you need to slow down a bit and learn, wanting to breed is one thing getting the basics ironed out at this point is a PRIORITY, that means proper husbandry, quarantine and make sure to have funds in the event something happens rather than buying more animals.