I have been creating apps with AppSheet for 4 years now, before Google's acquisition, and the sync button hasn't changed at all. I'm of the idea that if it ain't broke don't fix it.
However, all these years it is frequent that for new users you have to somehow explain to them that the orange numbers at the top right mean that the app is saving changes, because it is intuitive once you get it but not intuitive enough for a first time user.
I suggest updating the icon to an animated or dynamic one, or something that indicates the app is currently saving the changes, besides the numbers going down, instead of a static icon. Similar to the animation you get once you hit that sync button.