Our new, not yet released Rust-based desktop environment for Pop!_OS and other Linux distros is filling out with some essential systems that cater the DE to both users and developers alike. Customization is one of our main focuses for COSMIC, and was a huge focus for us in August, too.
Check out the application example in pop-os/libcosmic. You can also compare with all the examples in the iced-rs/iced repository. The iced API enforces that you model your application in an Elm style. Both APIs are under construction though. So breaking changes will happen over time. Though Rust makes it easy to refactor to changes.
Definitely agree from my initial experimentation about Rust making refactoring much easier. I’ll probably experiment some for fun while you all are working on the APIs. The sample app existing app projects are helpful and thanks for pointing me in the right direction.