Great points. However, I would argue that the year 2 leap made Zay "serviceable" and that the goal for the year 3 leap should be to become (at least) "reliable".
So far he is projecting to be the 3rd receiving option and I'm fine if whoever that guy may be is at least "reliable". Maybe that term means different things for different people, but he certainly was not reliable his rookie year and was up and down last year. Though you could attribute a lot of that to last year's offense in general.
Anyway, I think that's a fair floor for Zay right now, but he definitely has a higher ceiling. If he doesn't reach that, it won't be for a lack of trying.