To me this starts with my immune system well before i travel. I try to cultivate my gut microbiome so that I can have as strong an immune system as possible when I start out on a trip. I follow a vegan GI doc (Will Bulsewicz, MD) who also has a fiber supplement that encourages the growth of good bugs and try to eat 30 different plants per week to encourage gut microbiome diversity for health.
https://www.bbc.co.uk/food/articles/plant_points_explained
Having said that I will also say that I got a cold on my last trip. I masked when I was in the van (small group tour) but did not mask when I was outside or at meals. No one else caught my cough. I had been in Paris nearly 2 weeks before I joined up with my tour so exposure was not on the plane.
I had masked on the way to France with an N95. On the way home I did not mask and did not catch anything. I've been home 10 days. I also flew Delta One which I feel protects me a little just because the cabin is not so densely peopled and you are in your own little cubicle.
I'm so sorry you are ill. Take a look at the research on the gut microbiome and see if this is something you think might help you overall. BTW, one of the things you are encouraged to eat is fermented food. I can barely tolerate sauerkraut but I do try to eat a few spoonfuls several times a week particularly in the 4-6 weeks leading up to travel. I'm not sure how long it takes for good bugs to form but that is about all I can tolerate of fermented food!
Editing to add: I also use a nasal rinse. At home I use a Neti Pot but when I travel I use the Neil Med squeeze bottle. I take the packets of salt with me and get bottled water at my destination to use for the rinse. At home I used distilled but on a trip I just use bottled, still water. I do not use tap water. When I feel something coming on I will sometimes rinse AM and PM.