I wonder if a way to make a language gradual is to keep it primitive, but easy to build the more advanced features by composing them from primitive features. I mean, that's ultimately how all interesting programming is done, but it could be introduced at a much earlier and more basic level so it's not fearsome.
As an aside, I think that these online courses are a valuable resource, but it still helps to provide some human supervision, to make sure that a student hasn't fallen into a rut and given up.
I said this in my other reply, but if you can implement a language feature as a local rewrite, it does not add expressiveness. Features that add expressiveness must involve some sort of non-local transformation. A language feature adds expressiveness iff two programs that are equivalent in the base language in all evaluation contexts are not equivalent in the extended language. Felleisen came up with this idea in a paper, which Shriram Krishnamurthi explains in this video: https://pwlconf.org/2019/shriram-krishnamurthi/
As an aside, I think that these online courses are a valuable resource, but it still helps to provide some human supervision, to make sure that a student hasn't fallen into a rut and given up.