I’d say Flutter, a GUI framework backed by Google and it’s open source.

I’ve recently ported over a popular project called “llama.cpp” to Dart (language behind Flutter) and I’ve recently made YT video’s showing it running natively on macOS, Linux, Android, iOS, iPadOS and next up is Windows.

The official Ubuntu installer is made with Flutter too nowadays. But to be fair, last time I tried QT was somewhere in 2018, it might be a good option too.