13 year old cat with sinus problems

My 13 year old cat, Twinkles, has a nasal problem. He’s been treated over the past 2 years with different antibiotics( antirobe, clavamox). 3 months ago, my vet had a cat dentist come in & do an x-ray of his mouth & nasal area. They found the left eyetooth was bad and pulled it out, hoping this would alleviate the congestion & discharge from his left nostril. He went about 1 month, then the sypmtoms returned.

He currently runs a cycle of 1 week on Clavamox 2 weeks congestion free, then it starts over. Recently I found Richards Pet Wellness Tonic which has him holding off the congestion. I was to have a rhinoscopy done but finacially at this time $1300,00 is out of my budget. I feel guilty not being able to have this procedure done, but quite honestly, other then the nasal problem, he’s a happy & healthy. Any suggestions if another treatment regimen would be of benefit. ( I almost forgot: the x-ray revealed the absorption of the nasal bones in the left nostril, which was felt came from the infections off & on. Also most times nasal discharge is clear.)