As a PL designer, there is a lot to like here; the design seems quite reasonable and thought out.

But what I really like is the presentation: I like how all features are summarized, pointing out what is novel, and then everything is just discussed using examples and comparisons to other languages. This is something I'll have to try when I finally get my own language off the ground :).

I wonder if the author is interested in engaging with researchy PL designers. E.g. the testability orientation of the language would make for a good Onward paper, or maybe a Future of Programming workshop presentation.