Shrine is a great alternative to dragonfly and it's not coupled from Rails.
The new admin UI will be decoupled from the Rails app. We are investigating some techs like React.js or Stimulus.js. We already have the new admin design.
Refinery suffers an issue which is that it's coupled to the markup of its administration section. This complicates the code a lot and means that upgrades to things like new Rails versions are quite a lot more complex.
This project's aim is to turn Refinery into an API-first system which could have many admin theme implementations that suited different use cases.