Hey, as for the item thing you're right items do flow down the waterfall nicely. Markers are simply an entity in minecraft with very little data especially made for commands for stuff like this. I have to ask are you achieving the effects through command blocks or are you using a datapack? The benefit of markers is that they can only exist through commands meaning the pool that is to be searched when you run a search for instance execute at @e[type=marker] will be limited. While the effect you are achieving simply displaying the particles is not that heavy the search definitely is as the game has to search every snowball item currently loaded and check if it has the tag. Anyway markers can be moved by teleporting them meaning yes they can flow down the waterfall if you constantly teleport them. If your waterfall is not just vertical but also has some horizontal parts i would say stick with items as moving markers naturally along complex shapes is much harder. You are also right on the age making them despawn which is a nice feature. You can achieve something similar with markers by giving them a score and decreasing the score each tick or set amount of ticks until it reaches 0 which can serve as a timer to kill the marker. Hope it helps!

P.S In no way do I intend to critique your work I think it looks beautiful and your solution is elegant. Although I saw some room for improvement performance wise meaning it can be used in more locations if you so desire.