the only effective way to cure pyometra is to spey her, i uds ur concern abt her age. but the risk of general anesthetics depends very much on her kidney/liver's functioning abilities. long term anti inflammatory & antibiotics is not going to solve the problem, its only supressing the prob from going worse.
try consider Sx once again, or else more complicated problems are definitely going to happen (eg. pus goes from uterus to elsewhere)