Several years ago, I went down with a hardcore cold that just didn't want to end. For weeks and weeks. A lot of heavy, dry cough. (That was well before covid.)
After several weeks of such misery, I went to a specialist who tried to cultivate something from my sputum. Nope, normal flora. "That must be of viral origin," he said, "and the only thing I can recommend you is nasal irrigation with a plastic Rhino Horn. It worked well for many patients. Use this particular mineral water (Czech) and do it twice a day."
I already knew what nasal irrigation was and I was determined to avoid it for my entire life, but that illness brought me down to the "what do I have to lose?" condition. So I bought the horn and started flushing.
Yeah, it was awful at first, but my symptoms were gone 36 hours later. I started doing it regularly. My frequency of colds dropped significantly and so did their duration if they happen at all. I can sing better (some three semitones gained at the top of my range), I can breathe a lot better. And after the initial phase, where old nasty stuff was getting rinsed out of my sinuses, it is not even unpleasant anymore.
Can only recommend. My youth could have been a lot more healthy. I suffered from colds fairly often.