I’ve had this same problem and I’ve seen 4 ENT and 2 GI doctors trying to get relief to no avail. Tried saline rinses, nasal sprays and all kinds of medications for sinuses and GERD and nothing has worked. It seems like it could be something viral. I had Lyme along with the co-infections that come with it in 2020 and now have Post Treatment Lyme. Covid in 2021. The sinus congestion and coughing up thick phlegm didn’t emerge until 2023. I also had a visit with neurologist who told me it could be neurological but she didn’t know for sure.
Things slowly started to improve after I started new herbs to treat Lyme about a month ago. I’m trying Dr. Rawls RESTORE 180. I’ll come back here and comment if this works. It’s six month program and if this doesn’t help I don’t know what more I could possibly do.