I've wanted to do this task quite often, and I usually end up settling on something else that works. And in the case that I came across this time, I'll just draw a mask and animate it manually. BUT I would love to know if this is possible.
I had one idea, and it works to an extent, but it's not perfect. I added a gaussian Blur, then added levels with individual control. Make the alpha gamma zero, and animate the blur.
The problem with this, aside from making very stair stepped edges, is that it only works for objects that only have outside corners. Inside corners rounding gives a "melting" kind of look.
I really just want to evenly erode the entire alpha. Is that a thing and I just don't know what the effect is called? Doing that Should keep corners mostly intact.
Also, just to point out, this is a raster layer. Otherwise, yes, I could just a path offset.