sixcrows, I am so so sorry. I can so relate to this. Sinus headaches/infections can be debilitating. I recently started going to a different ENT doctor and he put me on a prescription irritation solution that helps me greatly. I also have had this mucous in the back of my throat like slimy saliva since COVID. Nothing I have tried gets rid of it. Salt water rinses help for a little while, and drinking a lot of water thins out mucous, but, I am loaded up again the next day. I am on prescription Mupirocin 20 mg/Budesonide 0.6 mg. which you mentioned and I irrigate my sinuses already with the NeilMed Sinus rinse daily. I have had about 6 sinus infections since COVID last year and then I had COVID again this past October. I have Long Covid and yes, there is a sinus connection. Singulair (prescription) did not help me at all. Covid has affected my sinuses greatly as I more prone to sinus infections. It also affected my asthma and I'm more prone to asthmatic bronchitis and pneumonia now. I never had pneumonia in my life until COVID last year. I get the "sick" sinus headaches as well that go into migraines for me. Sometimes doctors think it's a sinus headache when it fact it's migraine. I take Zomig (prescription) for regular migraines or sinus induced migraines and it helps greatly. Ask you doctor about this. I use heat packs and the Vicks steam electric inhaler unit (Amazon) that really helps. Most of the time with a sinus infection I am put on antibiotic and a steroid taper which will knock out the headache and opens up my sinuses.... have they tried that with you? I just had an MRI done and my sinuses are clear. I also use Afrin for 3 days only and come off if I feel like i am getting a sinus infection as you can get addicted to Afrin and rebound with it. I cannot take oral decongestants because I am on blood pressure medicine. I stopped the steroid sprays (Flonase, Nasonex, etc) and am using just the sinus irrigation. I have a hepa filter in my den and bedroom as I have a lot of allergies. Have you been allergy tested? I was just allergy tested today for environmental and food allergies. I have not been tested in about 8 years. Praying for you and I empathize. Keep me posted. I care.

Im sorry you are going though this. I understand, as Im post Covid, with very similar challenges. But this has affected my upper respiratory causing coughing and clearing of the throat all the time.

I too developed Extreme Post Nasal Drip. I started taking a supplement recommended by my chiropractor called Antronex made by Standard Process. It cleared it up in a couple of months, starting working in about a week, but took awhile to totally clear it up. I took 3 tablets morning and night until my symptoms seemed under control and then tapered down until now I only take 1 tablet a day or even every other day. Keeps it under control, soon I'll try to not take it at all and see if I don't need it anymore. Here is a link to a page on their website where you can put in your zip code and it will list locations where you can purchase it at.
https://www.standardprocess.com/where-to-buy
I believe it was $58 for the large bottle of 330 tablets, so not too expensive for you to try.