Anything for Linux? Or cross-platform?

I looked for Qt bindings but they appeared to be abandoned last time I checked.

GTK4 bindings https://github.com/chris-armstrong/ocgtk

I see that unison (file sync tool) seems to use: https://garrigue.github.io/lablgtk/

For Linux UI.

But that uses Gtk 3. Gtk 4 is already more than 5 years old ...

5 years is no age for an UI kit! ;-)