Marusan, I don't think full ceramic is a good idea on PPF as well because PPF is too soft and true ceramic coating forms a glass layer which is rigid. If all you want is slick surface and easy to clean please try out CarGuys Liquid Wax. It is very easy to apply and nothing sticks to it.
That was kind of my thinking, as well. I referred to it as the "crunchy coating."
There's also the fact that the film I chose (Llumar/Suntek) already has a hydrophobic layer. That said, I've been having a rough time with sap. It cleans off if you get it fast enough, but if it stays on too long, it can temporarily cause an issue in the film (it looks like it didn't get cleaned off). Fortunately, it seems to resolve after a day, so I supposed the self-healing layer is involved in that.