Is there actually active infection still - nasty discharge etc ?
If it is just her sounding snuffly and her lungs remain clear it may be inflammation as opposed to infection that is the cause. She probably has some scar tissue as a result of the Myxo nodules
If there is still active infection which is confined to her nasal passages then you could ask the Vet about trying Tiacil Eye drops. Sounds weird, but as long as the nasolacrimal ducts are draining the eye drops will drain into the nasal passages. Tiacil contains both an antibiotic (Gentamicin) and an anti-inflammatory (trometamol) which will act directly on the area of infection as opposed to doing so systemically via the blood stream.
This would give her a break from systemic abx