Nasal rinsing may help. In my case, it reduced my number of colds per year from 6 to approx. 1, and they also tend to be milder. This was likely accompanied by some change in nasal microbiome as well.
I'll look into this, maybe a salty flush. Logic is... people who live at sea or beach will 100 percent have a very saline/salty sinus cavity. So either salt sauna, or salt nasal rinse.