Nice project! I love Calibre's functionality, but I too was kind of frustrated by its UI and started designing an interface that kind of looks like a mixture of pre-Apple-Music iTunes and iBooks. I only created one screen so far, but feel free to take a look:
https://www.figma.com/file/m8qaYbPSnxiFPDPbuYtz2w/Calibre-re...
My main question was, how could Calibre's main functionality be presented in a UI that feels native to macOS?
Since this doesn't try to be cross-platform, I might try to build this as a SwiftUI app with your Rust backend if I find the time.
Kudos on actually publishing this! It might be hard to maintain something that keeps up with Calibre's development, but it sure seems like a fun hobby project :)