Unfortunately Apple has substantially more hoops to jump through in order to publish to the Safari extension store. Maybe in the future.

At least for the first few years, Apple kept changing the Safari extension tech enough that it wasn't worth keeping up with it. Has it finally stabilized to the point where you can build anything non-trivial and have it work for a few years?

I don't know. The main hurdle for me is the $99 / year fee that's required