How to keep glasses dry in the rain?
The easy answer is a wide-brim hat or long visor on your helmet. This does work, at least at low speeds. You will have to experiment with options, as nothing like this is sold for this purpose as far as I can tell.

Over about 20kph the length of brim you need gets silly, and by 30kph you need half a metre or so (there are some velomobile owners that do this). But if you're willing to slow down, the brim will work.

I wear glasses, and haven't found a water-shedding coating that works well enough to be worth the hassle. In practice I use a clip-on visor on my bike helmet, extended a little so it's about 10cm in front of my forehead. That keeps the worst of the rain off. Then I ride fast enough that my glasses get wet anyway. But not as wet as without it.

The visor/brim also reduces the amount of water running down my head and into my eyes, especially as that always seems to wash stale sweat out of the helmet pads and that stuff stings. A sweatband or headsock helps there.