I think Tauri would be the best solution for this app since Tauri is a lightweight alternative to Electron which plans to allow for compiling to Android and iOS in the future. And Tauri apps are written with web languages which is what Joplin is written in, the transformation would not be a complete rewrite.
Another good solution is Flutter which allows for apps to be compile for desktop and mobile but will require a complete rewrite of Joplin since Fultter uses Dart as its programming language.