Architecture
Components which are needed on the Server and on the Client.
The lightweight server software which provides the Interface for backup used by the clients
The pure Java Backup Client which uses the BackupServer Interface to backup, browse and recover files.
The Html Frontend for the server
This component contains everything that it can be used configured and monitored in an Synology NAS.
The sources for the installer for Windows OS
The sources for the installer for Linux OS
In generall Java should be the implementing language. Maybe Kotlin would be a possibility. Furthermore Java 11 would be a candidate because of the modularization. But currently I am not familar with it. But it depends what Synology supports. At least Java 8 is the minimum version.
The first idea is to setup a Spring Boot project
Extend the spring Boot project with Angular
A Java Swing Client which connects to Json Rest Services from the Backend
Currently I just know that there is a Synology Development Kit and that Java is supported.