To add on this while the Home Manager's module allows to configure KDE apps declaratively it is also impure, if I remove some sonfiguration from home.nix the configuration in the home directory remains, unlike with other programs that Properly respect the xdg division between config, cache and state.

Home-manager solved this by using kwriteconfig to write the corresponding config entries instead. But yes a dump tool would be nice to help migrate.

Speaking for nix, a bigger obstacle was the fact that for many applications config and state were mixed together in the same file, so we couldn't simply symlink that config file to the read only nix store

As an example implementation, here is how we implemented a similar functionality in home-manager https://github.com/pasqui23/home-manager/blob/e2e7ea9b8f3de1e6a09f4fc512eae75f6e413a10/modules/misc/qt/kconfig.nix#L19