Nose congestion? I feel like I've tried everything.

Our renal cat has had chronic nasal congestion since we've had him (over 7 years), long before his CKD diagnosis. It's varied in severity over the years, but nothing has really helped long-term. He's not FIV+ and other than CKD he doesn't have any diagnoses or allergies. If it was just impacting his nasal congestion I think we could keep that under control, but the issue is congestion > post-nasal drip > vomiting mucus between 4-7am almost every night. I've consulted his vet and it's been a mess of troubleshooting for years with very little success, and I just want to make sure I'm not missing some miracle solution.

Things I've tried with the consultation and approval of his vet:

• Antibiotics - did nothing
• Various antihistamines - seemed to work for a couple of days, then did nothing
• Steam - very temporary relief
• Nebulizing with pure saline - effective at getting things loosened up, not great at getting things moved out
• Pure saline drops in each nose - effective at moisturizing his nasal passages, but not much else
• Aspiration with a hospital-grade children's nasal aspirator - very effective at getting what's immediately in his nose, not effective at preventing the post-nasal drip vomiting situation

He's on a very strict schedule of other meds and supplements to address his CKD, nausea, digestion, control stomach acid, stimulate his appetite, etc. and all of them seem to be working to their intended purpose, leaving his congestion as the top offender. I've also considered that the issue could be dental, but a visit to a feline dental specialist confirmed there's no association. I also have him on a preventative dental routine to be sure it stays that way.

Any advice or help would be so appreciated!