but from previous experiences like this, the feature set has to be robust at the start or I think adoption will suffer.

Refinements themselves present a significant hurdle to adoption by virtue of their limitations and overall introduction of conceptual complexity. So it’s a tough sell to recommend this for anything outside of personal projects or places with incredibly strong esoteric Ruby knowledge (like, say, hidden away within Rails).

It is very important that "production ready" UI libraries are available because otherwise the use of Svelte cannot be argued. A key point would be to make it easy possible that people can contribute