I used to religiously Rain-X my windshields and then got tired of doing it. It does work well if you maintain the coating. I've often wondered if a thin coating of parylene or FEP on glass would be a long-lasting repellent coating but it probably doesn't last long with dirt / dust / abrasion...
This got me wondering as to what do they use on aircraft? Found this:
https://www.ppgaerospace.com/getdoc/ff939dfe-b0b3-4401-90ed-9d054d604af8/Surface-Seal%C2%AE-Coating-Quick-Application-Kit.aspx
keywords: hydrophobic glass coating
Some others:
https://www.aircraftspruce.com/catalog/cspages/rainbgon.php?clickkey=134872
https://www.nanotechcoatings.com/automotive-coatings/hydrophobic-glass-coating/
https://www.griotsgarage.com/glass-sealant-8-ounces/
I haven't tried any of these. Visibility in my truck in the rain is awful; I might give some of these a go...