The two main languages that can work on multiple platforms is Java and C#.
I know that you have put a lot of work into developing TestController in Java, but while Java is open standards based and cross-platform the Java platform has been focusing on server-side (web) applications for many years, leaving Desktop application support behind, combined withthe problems associated with requiring separate installation of the Java runtime (JRE).
If I were to develop a similar cross platform desktop application today, I would probably base it on the Electron framework that is designed to create crossplatform desktop applications, is open source and can provide a single (monolithic) installer on all common platforms (Windows, Linux, MacOS).
The underlying runtime platform is Node.js and the rendering is done using an embedded Chromium browser.
The list of applications written using the Electron framework (notably VS Code), and the available development and documentation resources means that it is likely to continue to exist and be maintained for many years into the future (see
https://en.wikipedia.org/wiki/List_of_software_using_Electron )
Alternatively, one could use PYTHON with a suitable GUI framework, but that would still leave the problem of a separately installed runtime with all the trouble it entails.